Most Recent Weekly Report: 7 May-13 May 2014

PVMBG reported that during January-8 May diffuse white plumes rose 25-50 m above Anak Krakatau. Seismicity continued to be dominated by shallow and deep volcanic earthquakes, as well as signals indicating emissions. The Alert Level remained at 2 (on a scale of 1-4). Residents and visitors were warned not to approach the volcano within 1 km of the crater.

Most Recent Bulletin Report: December 2012 (BGVN 37:12)

September 2012 eruption spews ash; lava flows reach the sea

One of our previous reports on Krakatau (BGVN 36:08) discussed two eruption episodes, one spanning from 25 October 2010 to March 2011 and another beginning in August 2011 and continuing through the end of that report, around 1 October 2011. During the last two weeks of September 2011 volcanic earthquake swarms and diffuse emissions persisted. In November 2011, the photographer and guide Øystein Lund Andersen visited Krakatau and observed mild Strombolian explosions (BGVN 37:11).

This report summarizes behavior chiefly during 1 October 2011 through early October 2012. Eruptions around early September 2012 deposited ash on towns in Sumatra, and lava flows extended the shoreline of the island (Anak Krakatau) by ~100 m. The Alert Level was lowered after that and remained low into at least mid-January 2013.

Figure 30 is an index map showing the location of the famous caldera Krakatau, located in the Sunda Strait (E of Sumatra and W of Java). The smaller feature Anak Krakatau grew to form an island well after the 1883 eruption and continues as the active center.

Figure 30. An index map showing the location of Krakatau. Small yellow triangles indicate other Holocene volcanoes (from the current Global Volcanism Program database). Created by GVP staff from ARC software.

October 2011-2012. The Alert Level was raised to 3 (on a scale of 1-4) on 30 September 2011, prohibiting visitors and residents from approaching within 2 km of the active crater. On 8 October 2011, a Jakarta Post article stated that activity at Krakatau was increasing; the number of seismic events on 6 October was 5,204; on 7 October, 5,543; and on 8 October, 5,883.

On 26 January 2012, CVGHM lowered the Alert Level from 3 to 2, noting that the number of tremor events had decreased significantly. The lowered Alert Level excluded visitors and residents from approaching within 1 km of the active crater.

During 1 June-1 September 2012, CVGHM’s visual observations were often prevented by fog cover. When views were clear during June, observers saw occasional diffuse white plumes above the crater. In June, July, and August, the respective seismic events totaled 1,075, 807, and 2,335. Detailed seismicity during 1 June-2 September 2012 is cataloged in table 8.

Table 8. Type and occurrence of earthquakes and tremor at Krakatau during 1 June-2 September 2012; ‘-’ indicates data not reported. Courtesy of CVGHM.

Seismicity increased on 2 September 2012. CVGHM recorded continuous tremor, and a Strombolian eruption ejected lava 200-300 m above the crater. At times, residents heard booming sounds that rattled windows. On 3 September, the Darwin Volcanic Ash Advisory Centre (VAAC) reported that ash plumes rose to altitudes of 2.4-4.3 km and drifted 35-95 km N. The Jakarta Globe (citing the state news agency Antara News) reported that a dense ash plume had drifted N, reaching a number of areas in Lampung (the most southerly province of Sumatra), and blanketing Lampung’s capital, Bandar Lampung (~75 km NNW of Krakatau; figure 31), with a thin layer of ash.

Figure 31. Sketch map showing Krakatau and Sumatra’s Lampung province (Indonesia) including the capital, Bandar Lampung, the scene of September 2012 ashfall from the post-1927 vent, Anak Krakatau (the small island labeled with a triangle). Adapted from a brochure offered for tourists (Indonesia Destination & Travel Information Guide; baliwww.com).

Ashfall on 2 September prompted officials to recommend that residents and tourists wear masks outside and not venture within 3 km of the volcano. The Jakarta Post indicated that government officials planned to distribute 680,000 masks to residents in a number of affected districts in anticipation of further Krakatau explosions.

According to Antara News, less intense tremor continued on 4 September. A satellite image acquired by NASA’s Earth Observatory on 4 September showed fresh lava flows descending Anak Krakatau’s SE flank, extending the shoreline by about 100 m (figure 32).

Figure 32. A natural-color satellite image of Anak Krakatau acquired by the Advanced Land Imager aboard Earth Observatory-1 on the morning of 4 September 2012. The NASA caption noted that fresh lava flows extended the SE shoreline by ~100 m. The ash plume drifted W. Courtesy of NASA Earth Observatory.

September 2012 eruption and visit by Volcano Discovery. A Volcano Discovery group toured Krakatau during the first several days of September 2012 and observed what they noted was the largest explosion there in ten years. They noted that seismic activity (recorded by CVGHM) peaked on 2 September, with a day of continuous explosions and lava descending the volcano’s E and W flanks. Photos showed lava entering the sea (figure 33).

Figure 33. A lava flow from Anak Krakatau reaching the sea on 3 September 2012. Courtesy of Volcano Discovery.

The Volcano Discovery group noted that the eruption had altered part of the S and W crater rim, splitting the rim into two parts. They saw an abundance of lava bombs on the volcano’s flanks in the forest, indicating the force of the explosions. The group indicated that by 5 September, activity had greatly diminished and incandescence from the crater was absent. On 6 September, Krakatau was calm with limited seismic activity reported by the observatory.

On 8 September, the Jakarta Post also reported that seismic activity had greatly diminished during the previous two days. However, this news account noted that residents in Bandar Lampung still reported ashfall. The article also stated that ash from the eruption had damaged volcano-monitoring equipment.

The Alert Level remained at 2 from 26 January 2012 through at least 3 January 2013.

2012 visits by Øystein Lund Andersen. As indicated in our previous report on Krakatau (BGVN 37:11), the photographer and guide Øystein Lund Andersen has visited Krakatau multiple times and his website contains good descriptions and photos of the volcano during these visits. The following describes his observations during 2012.

On 8 January 2012, during a 3-hour visit, Andersen observed no Strombolian activity, in contrast with his observation on 13 November 2011. On his next visit, during 12-14 February, he reported medium to heavy venting from the crater and fumarolic activity that was more intense than the activity during January. He noted continuous steam-and-gas emissions that rose 100-500 m and incandescence at night, but no eruptions.

During a visit on 6-7 April 2012, Andersen noticed that the S part of the crater was illuminated at night. He further reported that on 7 April, Krakatau started to produce small eruptions from the S part of the crater, the same side as the growing lava dome.

During 6-8 May, Andersen noted that activity at Krakatau had decreased somewhat in the previous several weeks. Steam plumes reached a height of 100-200 m and seemed less intense than during his visit in April. He noted incandescence at night, but it was less intense that the previous month. Andersen reported on 7 April: “footage taken by Pierre Fortine showed no sign of any lava dome, but the red glow that is often clearly visible at night from Verlaten, Lang, or Rakata are in fact multiple glowing vents (some of them were gas vents that were burning) and red hot material surrounding them. The lava dome that people have claimed to observe [through] February to April may have been destroyed during the last small eruptions that I reported of in April. Shallow earthquake data recorded by the Krakatau Volcano Observatory in Pasauran (PVMG) shows that the level of activity remains on the relative same level as last month.”

During 2-3 June, he reported that activity had decreased since April and May. He stated that incandescence was almost non-observable, and steam plumes only rose 50-100 m on this visit (compared to 200 m in May), and were at times non-existent.

On 3 September 2012, Andersen wrote that he had heard continuous booming noises half way from Java, and that large booming sounds could be heard in the villages of Carita and Anyar (or Anyer), neighboring villages on the W coast of Java, about 50 km ESE of the volcano. As he approached the volcano, he noted a high plume from the main vent and the ejection of lava bombs to heights of up to 300 m. An area on the SE shore also emitted a large steam plume. According to Andersen: “The local crew/guides who joined our group looked very surprised and worried, as we all noticed these major changes. I first thought this was the result of new geothermal activity, but first realized later that this was in fact a new lava flow.” According to Andersen, the lava flow had extended the seashore on the E side by up to 100 m, and the E and W part of the crater walls had experienced a partial collapse (figure 34).

Figure 34. New lava extending the seashore on the E side of Anak Krakatau on 3 September 2012. Courtesy of Øystein Lund Andersen.

On a visit during 6-7 October 2012, Andersen observed no activity other than a weak and irregular steam plume and, at night, “some small spots of glowing lava, near and on the lava flow on the western flank.” On this visit, he studied the new lava flow. He reported that the lava had flowed down both the W and E sides of the volcano, leaving deep scars on both flanks. According to Andersen, news accounts had reported the lava flow on the E flank, but few had noted the one on the W flank, which was significant, although not as great as the one on the E flank.

Weekly Reports

7 May-13 May 2014

PVMBG reported that during January-8 May diffuse white plumes rose 25-50 m above Anak Krakatau. Seismicity continued to be dominated by shallow and deep volcanic earthquakes, as well as signals indicating emissions. The Alert Level remained at 2 (on a scale of 1-4). Residents and visitors were warned not to approach the volcano within 1 km of the crater.

Based on a pilot observation, the Darwin VAAC reported that on 31 March an ash plume from Anak Krakatau rose to an altitude of 3 km (10,000 ft) a.s.l. and drifted N. Ash was not identified in satellite images.

CVGHM reported that during 1 June-1 September observations of Anak Krakatau were often prevented by fog; occasionally diffuse white plumes were observed rising from the crater in June. Seismicity increased significantly in August. On 2 September seismicity again increased, and at 1830 a Strombolian eruption ejected lava 200-300 m above the crater. The Alert Level remained at 2 (on a scale of 1-4). Residents and visitors were warned not to approach the volcano within 1 km of the crater.

Based on analyses of satellite imagery, the Darwin VAAC reported that on 3 September ash plumes rose to altitudes of 2.4-4.3 km (8,000-14,000 ft) a.s.l. and drifted 35-95 km N.

On 8 October, a news article stated that activity at Anak Krakatau was increasing; the number of seismic events was 5,204 on 6 October, 5,543 on 7 October, and 5,883 on 8 October. The Alert Level remained at 3 (on a scale of 1-4), and visitors and residents were not permitted to approach the volcano within a 2-km radius.

CVGHM reported that seismicity from Anak Krakatau in 2011, as late as 10 July, consisted of 20-30 volcanic-earthquake events per day and shallow events ranged from 120 to 135 events per day. Hundreds of events per day were detected during swarms. On 10 July, the seismic equipment was damaged by Krakatau's activity but was again operational in mid-September. During 18-30 September seismic events reached 4-5 events per minute. Visual observations in 2011 until 13 September indicated occasional explosive eruptions that would eject material and produce ash plumes. During 14-30 September fumarolic activity from the crater and in the fumarolic fields was visible. The Alert Level was increased to 3 (on a scale of 1-4) on 30 September based on an increase in seismic activity and widespread fumarolic activity.

Based on information from CVGHM, the Darwin VAAC reported that on 12 January an eruption plume from Anak Krakatau rose to an altitude of 2.4 km (8,000 ft) a.s.l. and drifted SE. Meteorological clouds prevented observations of the area from satellite. On 15 January, a pilot observed a plume that rose to an altitude of 3 km (10,000 ft) a.s.l. and drifted 18-28 km E.

A news report on 2 November noted that the frequency of explosions from Anak Krakatau had slowly increased to 100 per day since 25 October. During 31 October-1 November there were 251 explosions recorded.

A news report on 18 June noted that activity at Krakatau had increased significantly. According to the head of the volcano monitoring post in Pasauran village there were 828 small eruptions in the previous six days, reaching the rate of a new explosion every three minutes. Observers on beaches in Java could clearly see rising white gas-and-steam plumes along with incandescent ejecta at night. Residents also reported loud explosion noises. The level of activity decreased again on 19 June, and the Alert Level remained at 3 (on a scale of 1-4).

CVGHM reported that the number of eruptions from Anak Krakatau increased significantly at the end of March and continued through 5 May. Seismic data were not collected during 26 April-29 April due to instrument malfunctions. Direct observations of the crater on 24, 25, and 29 April revealed that the eruption originated from a crater on the W slope of Anak Krakatau. Ash plumes generally drifted E and caused ashfall within a 5 km radius of the crater. Clear weather on 5 May allowed for visual observations; "smoke" rose 500 m above the crater. On 6 May, the Alert Level was raised to 3 (on a scale of 1-4).

According to a news article on 29 April, some residents on western Java (Lampung) near Krakatau have evacuated due to their observations of increased volcanic activity during the previous week. Observers reported loud blasts, lava flows, and ash plumes that rose 200-800 m above the Anak Krakatau crater. Pilots had also reported seeing ash plumes. A volcanologist from CVGHM stated that the activity did not merit an increase in the Alert level. The Alert Level remained at 2 (on a scale of 1-4).

During 27-30 March and 1 April, CVGHM reported that visual observations of Krakatau when the weather was clear revealed that ash plumes rose 200-800 m above the Anak Krakatau crater. On 2 April, an ash eruption was seen on satellite imagery and reported by a pilot. A resultant ash plume drifted more than 60 km S. The Alert Level remained at 2 (on a scale of 1-4).

According to a news article, explosions and earthquakes from Anak Krakatau averaged 120 per day approximately during 11-17 August. Monitoring personnel observed active lava flows, ejecting rocks, and emissions of "smoke."

According to a news article, eruptions from Anak Krakatau increased in frequency during 10-11 August. On 12 August, monitoring personnel reported that active lava flows and emissions of thick "smoke" continued but that the frequency of earthquakes and eruptions had declined.

Based on observations of satellite imagery, the Darwin VAAC reported that ash plumes from Anak Krakatau rose to an altitude of 3 km (10,000 ft) a.s.l. on 27 July and drifted NW. A meteorological cloud obscured satellite views the next day but the VAAC warned that ash may still be present in the area.

CVGHM reported that during 22 June-1 July, the number of seismic events from Anak Krakatau decreased significantly and booming noises were less frequently heard. During 1-3 July, ash emissions also declined. Based on a pilot report, the Darwin VAAC reported that a low-level plume drifted NW on 2 July. On 3 July, CVGHM lowered the Alert Level to 2 (on a scale of 1-4).

Based on observations of satellite imagery and pilot reports, the Darwin VAAC reported that a low-level ash plume from Anak Krakatau rose to an altitude less than 3 km (10,000 ft) a.s.l. on 1 July and drifted NW.

CVGHM reported that seismicity from Anak Krakatau increased during 14-21 April; the number of events per day peaked on 20 April. Ash plumes accompanied by propelled incandescent rocks were noted during field observations on 16, 17, and 18 April. The eruption affected the summit and the E and S flanks. Booming noises were reported and occasionally heard at an observation post 42 km away. The Alert Level was raised to 3 (on a scale of 1-4) on 21 April.

CVGHM lowered the Alert Level for Anak Krakatau to 2 (on a scale of 1-4) on 1 April. Seismicity declined in early February, and eruption plumes and propelled incandescent material were not seen after 4 February. Visitors and residents were advised not to go within a 1.5-km radius of the summit.

According to a news article, incandescent rocks erupted and plumes from Anak Krakatau rose to altitudes of 2.8-3.3 km (9,200-10,800 ft) a.s.l. on 20 January. Eruptions reportedly had a "deafening sound" and could be seen from Sertung and Rakata islands.

According to news articles, gas-and-ash plumes from Anak Krakatau continued rise and seismicity was elevated during 9-11 November. Incandescent material was propelled from the summit and fell onto the flanks. Lava flows were also observed traveling down the flanks. Villagers and tourists were advised not go within a 3-km radius of the summit. The Alert Level remained at 3 (on a scale of 1-4).

CVGHM raised the Alert Level to 3 (on a scale of 1-4) for Krakatau on 26 October due to the presence of multiple gray plumes from Anak Krakatau and an increase in seismicity during 23-26 October. Plumes rose to an altitude of approximately 1 km (3,300 ft) a.s.l. during 23-26 and 30 October. Inclement weather resulted in only intermittent observations. Villagers and tourists were advised not go within a 3 km radius of the summit.

Due to a decrease in seismic activity at Krakatau over a 4-day period, the Alert Level at the volcano was reduced from 2 to 1 (on a scale of 1-4) on 19 April. The volcano was considered to be at a normal level of activity. Visitors were still banned from the summit and crater of Anak Krakatau due to toxic gas emission.

In July there was an increase in the number of gas-and-steam emissions from the crater of Anak Krakatau. In addition, the number of volcanic earthquakes increased from 1-4 per day before 5 July, to ~57 per day, then dropped to 2-17 earthquakes per day by mid-July. The Alert Level at Krakatau was raised from 1 to 2 (on a scale of 1-4). Visits to the crater were prohibited.

During 24-30 March there was a significant decrease in the number of earthquakes at Krakatau in comparison to the previous week. The number of shallow volcanic earthquakes decreased the most; 58 occurred during 17-23 March, whereas 12 occurred during the report period. Krakatau remained at Alert Level 2 (on a scale of 1-4).

During 17-23 March, seismicity at Krakatau was dominated by volcanic earthquakes and there was a significant increase in the number of shallow events. No visual observations could be made due to inclement weather. Krakatau remained at Alert Level 2 (on a scale of 1-4).

Volcanic activity at Krakatau was at low levels from May until mid-August when the number of daily seismic events increased. There was no corresponding increase in surface activity. The Alert Level was raised from 1 to 2 (on a scale of 1-4).

The number of explosion earthquakes decreased significantly during 10-16 September in comparison to the previous week, while the number of small explosion earthquakes increased. The volcano remained at Alert Level 2.

In comparison to the previous week there was a significant increase in explosion and volcanic earthquakes at Krakatau during 3-9 September. The number of small explosion earthquakes sharply decreased. The volcano remained at Alert Level 2 (on a scale of 1-4).

There were 728 shallow volcanic earthquakes at Krakatau during 9-15 July, which was a significant increase in comparison to the previous week. No visual observations were made. The volcano remained at Alert Level 2 (on a scale of 1-4).

Anak Krakatau showed a significant increase in activity during the week of 12-18 March. The number of shallow volcanic earthquakes (type B) rose to 79 from 25 the previous week. Activity decreased again during 19-26 March, with only 34 shallow volcanic events. Krakatau is at hazard level 2 (on a scale of 1-4).

Bulletin Reports

All information contained in these reports is preliminary and subject to change.

09/1972 (CSLP 61-72)Large eruption plume on 26 June

Card 1449 (26 September 1972) Large eruption plume on 26 June

"Max Sons . . . was a member of a group in Indonesia. Shortly before noon local time on 26 June 1972, the group was returning from Sumatra to Djakarta in a DC-3 aircraft and circled Krakatoa to take pictures. As they were pulling away heading toward Djakarta the volcano came alive, with a classic mushroom cloud quickly forming above the island. In 10 minutes the column rose to perhaps 10,000 feet, and by the next day it could be seen 50 miles away, as the group flew from Djakarta to Singapore."

Information Contacts:Max Sons, Standard Oil Company.

10/1972 (CSLP 61-72)Underwater eruption in April; continued activity through September

Card 1466 (16 October 1972) Underwater eruption seen along S coast on 11 April

"11 April 1972 at 0330 GMT the vessel was passing 9 miles S of Anak Krakatau (say 6°18'S, 105°26'E [from Durban towards Djakarta]) when we observed a disturbance on the south side of the island which at first looked like breakers. On closer observation it was seen that an underwater eruption was taking place all along the south side of the coast and gases were also seen to be given off from this disturbance. Smoke was also observed from fissures on the peak of the island. The smoke started at about 100 feet from the summit up to about 400 feet, and a considerable amount of white ash was seen on the southwest side of the island." (Captain Rodger, W. Sinclair, 2nd Officer, C. McConachie, 3rd Officer, Cadets R. Lawrie and L. Macintosh.)

Card 1467 (17 October 1972)Review of activity during June-September

"From the second week of June 1972, ash eruptions from the active cinder cone Anak Krakatoa in the center of the island group took place with time intervals of 15 minutes, 30 minutes to one hour. Ash clouds, white to grey colored, reached heights of 50-100 m causing ash showers depositing thin layers of whitish ash in the vicinity, while bigger eruption material fell back into the crater. Weak rumblings accompanied the eruptions, and could be heard from time by people along the coast of west Banten. In the night fire could be observed in the eruption clouds. The activity . . . lasted during the months of June, July, August and September. According to inhabitants of Carita village (west coast of Banten), the activity diminished in September and stopped completely at the end of the month. According to a report by a tourist who visited the volcano on 1 October from 1000 to 1200, no activity was observed. Only fire could be seen in a small crater within the central crater.

"This recent eruption . . . was limited to small ash eruptions accompanied by weak rumblings, which lasted for almost four months. This kind of rather quiet activity is very common for this volcano . . . which came into existence in 1929 and grew bigger by the day: recent height over 170 m. To people living on the coastal areas of western Java and southern Sumatra the activities are more or less routine and do not cause fear at all. As the activity of the volcano is considered harmless (at least for the present time), the Volcanology Division currently does not carry out continuous observations, but makes only incidental investigations on the spot to record remarkable changes of the active cinder cone, topographical changes of the island, changes of chemical composition of eruption products, etc."

The following extract is from the meteorological logbook of MV Dardanus, Captain D.M. Belk, on passage Durban towards Djakarta.

"21 July 1972 between 1345 and 1530 GMT. The vessel was approaching the Sunda Strait and the sky in the northeast had a reddish tinge, varying in intensity. Soon it was observed through binoculars that it was an island throwing hot ashes and lava into the sky. At first the island was thought to be Rakata . . . but it was difficult to ascertain as it was still 25 miles off. When the vessel was 5 miles off Rakata it was observed that the island erupting was . . . a small island about 2.5 miles north of Rakata and about 510 feet high.

"The eruptions lasted for periods of 20-30 seconds at intervals of 2-7 minutes. When see through binoculars, the whole island seemed to be ablaze for 40-50 seconds after each eruption. The estimated height of the ashes and lava being thrown into the air was about 500 feet, estimated by the height of the island itself. When to leeward . . . the smell of burning was strong but not much smoke was encountered. Wind SSE force 2." (J. Dixon, 3rd Officer.)

An eruption from a summit crater of Anak Krakatau (figure 1) began on 10 July and was continuing in October. Lightning over the summit was seen from a nearby village on 10 July and small amounts of basaltic ash were ejected. Other explosions occurred on 14, 18, 21-23, and 30 July, and on 3 August. During the largest explosions (on 14 and 22 July) tephra clouds, including some bombs, rose 400-500 m above the crater. Activity was confined to vapor emission 11-13, 15-17, and 26-29 July.

Figure 1. Map of Anak Krakatau, 1978. Courtesy of VSI.

VSI scientists visited the volcano 21-23 July. Tephra emission occurred at intervals of 15 minutes to 6 hours. Ash was always ejected by the explosions but larger tephra was only occasionally present in the eruption clouds.

The eruption had declined by the third week of August but had returned to July levels when the volcano was revisited 2-5 October. Activity was Strombolian, consisting of discrete groups of explosions. Each explosion group lasted an average of 9 minutes, with an observed time interval between first explosions of successive groups ranging from 5 to 27 minutes. The first explosion of each group was always the largest, typically ejecting bombs 75-100 m above the crater. Some bombs fell back into the crater and others described parabolic arcs, falling 300-400 m away and forming impact craters averaging 40 cm in diameter and 10-15 cm deep. Dark gray, ash-laden, cauliflower-shaped clouds ejected with the bombs rose 200-400 m at an average velocity of 5 m/second. Lightning was visible in the ash clouds. Coarse ash fell 500 m from the crater and finer material was blown into Sunda Strait. Water vapor was emitted from cracks and fissures formed along the inner wall of the active crater. Vapor emission appeared to increase 3-5 seconds before the first explosion of each group.

A single component (vertical) Hosaka electromagnet seismograph recorded 554 explosion earthquakes during 77 hours of observation, using a 0.3 second transducer 1 km from the crater. Using the minimum amplifier magnification (about 2,000x) a maximum double amplitude of 15 mm was recorded.

Activity from Anak Krakatau's 100-m-diameter 1978 crater resumed in mid-July. Bombs (average diameter 1 m), lapilli, and ash were ejected every 5-15 minutes, rising 200 m and covering the area within about 700 m of the crater. Lava flowed W, reaching the coast about 450 m away. A danger zone has been delineated within 3 km of the crater.

The 1979 eruption is stronger than that of 1978, when ash and lapilli were ejected, but no bombs or lava flows.

Lava extrusion had ended by early September, but tephra emission continued. Activity fluctuated during ten days of observations in early and mid September, but usually consisted of discrete explosions at intervals ranging from 20 seconds to 40 minutes. Ash clouds rose to as much as 2 km above sea level and incandescent tephra formed fountains that reached several hundred meters height. Some of the explosions were audible up to 50 km away. Activity continued at the end of September, with ejection of 1-2 m bombs and finer pyroclastics taking place about every 2.5 minutes.

Maurice Krafft visited Anak Krakatau 5-8 and 13-15 September and flew over the volcano for 3 hours on 12 September. A 3-man team (Rudy Hadisantono, Stephen Self, and Michael Rampino) investigating the products of the 1883 eruption observed the volcano 10-12 September.

The following is from Maurice Krafft.

5 September: three vents, aligned NE-SW, were active in the 1978 crater. The SW vent emitted clouds that rose 200 m; an ash cloud rose 300 m from the middle vent; and incandescent bombs (average diameter about 0.5 m) from the NE vent reached 400-500 m in height, covering the area within about 1,000 m of the vent. Ash and gases from the NE vent rose 1,000 m. Explosion frequency averaged one every 3-4 minutes.

6 September: The SW vent had become quiescent, but explosions from the other two vents occurred every 5-10 minutes. Bombs reached 400-500 m above the crater and fell as much as 1,200 m away, on the E end of the island. At about 2100, activity began to weaken, and continued to decline during the night. Explosions on 5 and 6 September were heard within 50 km of Krakatau, on Java and Sumatra.

7 September: Only the NE vent remained active. An eruption cloud containing considerable ash but very few bombs was ejected every 20-30 minutes, rising about 500 m. Most of the explosions were not audible, but noisy explosions ejected bombs at about 2-hour intervals.

8 September: Weaker activity; explosion frequency declined to every 30-40 minutes and audible events that ejected bombs were less common than the day before.

The following is from Stephen Self.

"During the night of 9-10 September, the explosions could be heard, accompanied by volcanic tremor, on mainland W Java, 45 km from Krakatau.

"The activity observed on 10 and 11 September consisted of periodic explosive ejection of juvenile bombs, non-juvenile lithic blocks and large amounts of fine ash. The interval between explosions varied from 20 seconds to 20 minutes with no obvious pattern of periodicity. The explosions were often frequent enough to maintain an eruption column of fine ash and gases to a maximum of 2,000 m above sea level; winds blew the column WNW.

"The estimated initial volocity of the most powerful explosions was 150-170 m/second based on timed plume-rise during the gas thrust phase. Convective rise velocities varied from 10 to 20 m/second; large blocks (1-2 m diameter) were ejected into the sea up to about 1 km from the active crater.

"At night, the ejecta were incandescent, forming spectacular lava fountains up to 200 m above the vent. The activity, therefore, has the characteristics of Strombolian explosions, but produces much more fine ash than in more basic Strombolian activity.

"A new cone has been built around the 1978 crater and reaches about 150 m above sea level. At times, deposition on the cone was so heavy that it was 50% coated by glowing bombs at night. Fine gray deposits were accumulating on the older islands of the Krakatau group, with a total of 3 cm on the N and central parts of Sertung Island (about 5-6 km NNW of the crater).

"On the afternoon of 11 September, the activity had dwindled to occasional weak convective plumes and the team landed on Anak Krakatau. They ascended the 1930-40 crater rim on the E side of the island and collected fresh bombs ejected from the active vent (on the W side of Anak Krakatau). The bombs were andesitic and varied from massive and glassy to poorly vesicular lava with a plagioclase phenocryst content of 15-20%. The phenocrysts were up to 3-5 mm in length.

"The coarse ejecta were purely magmatic and it appeared that there was no contact between sea water and the rising magma. However, the large quantity of fine ash may suggest some phreatomagmatic mechanism.

"The team last observed the volcano late on 12 September, when the activity was slightly less than on 10 and 11 September."

The following is from Maurice Krafft:

12 September: A 3-hour morning overflight revealed explosions every 4-5 minutes, producing 800-m-high ash clouds. After some explosions, bombs fell into the sea at the W coast of the island.

13 September: Explosions occurred at about 20-minute intervals. Ash clouds were voluminous and rose about 1,500 m, but few bombs were ejected.

14 September: Activity was similar to the previous day. In addition, a considerable amount of lightning was observed in the ash clouds.

15 September: Explosion frequency dropped to one each 30-40 minutes, but ash clouds continued to rise 1,000-1,500 m.

The following is from Adjat Sudradjat. At the end of September bombs 1-2 m in diameter fell as much as 400 m from the crater, and finer pyroclastics fell as much as 700 m away. Two eruption columns were visible, indicating that there were two active vents. Quiet intervals between explosions were about 2.5 minutes long.

An eruption of Anak Krakatau began in mid-July from the 1978 crater (see figures 1 and 2), ejecting tephra and extruding lava that flowed into the sea at the island's W coast. Lava extrusion had ended by early September, but tephra emission continued at varying levels of intensity through the end of the month.

Figure 2. Oblique airphoto of Anak Krakatau, 12 September 1979, looking approximately NW. Verlaten Island (caldera rim) is in the background. An eruption column rises from the 1978 crater. Photo taken by Maurice Krafft.

The average number of eruption tremors/day recorded at Pasauran, 30 km from the volcano, was 70 in July, 50 in August, and 100 in September, then dropped sharply to two in October and November. No tremors were recorded in December, and detonations and ejection of incandescent materials were no longer observed.

Activity began to increase at the end of March. Detonations from explosions were heard 50 km away and window glass trembled on the W coast of Java. Incandescent material rose 200 m above the vent, which approximately coincided with the 1975 eruption center. About 65 explosion events were recorded on 13 April, and 290 on 16 April. The strongest activity occurred during a 5-hour period on 19 April, when 200 explosions were recorded. There were 335 explosions on 20 April, but activity was declining the next day. Rough seas prevented a landing on the island.

Increased activity from Anak Krakatau began in March, when detonations were heard from Pasauran, 40 km away on the W coast of Java. Incandescent material was thrown to 200 m height during a period between 19 April at 2300 and 20 April at 0415, when 200 explosions were recorded. Activity declined after 20 April but continued intermittently through September.

A stronger explosion occurred on 9 September at 0039. It rattled windows and shook houses in Pasauran, and a 3-4 cm-amplitude explosion event was recorded on the seismograph there. Ash clouds reached about 1.5 km altitude in September and explosions continued at the end of the month. The source of the 1980 activity is a new vent that approximately coincides with the 1975 crater and is about 250 m NW of the 1978-79 eruption center.

A research group consisting of a biologist, oceanographer, environmentalist, and volcanologist are studying Krakatau under the auspices of the centennial commemoration of the 1883 eruption. The Centennial Committee invites scientists worldwide to participate in a 3-year period of research at Krakatau.

Explosions resumed 20 October after several months of fumarolic activity. Guy Camus and Pierre Vincent visited the volcano for four hours during the afternoon of 19 October, but noticed no premonitory activity. Explosions began between 0300 and 0400 the next morning. From Rakata Island (about 3 km SE of Anak Krakatau), Camus and Vincent noted 19 explosions in the two hours just after sunrise, before leaving the island. They had seen several others by mid-afternoon during discontinuous observations from a boat. Most were initiated by a "cannon-like" explosion from the main cone, followed by convective growth of an eruption column (typically to 400-600 m, but occasionally to [2] km in height). No noise could be heard on Rakata Island. The explosions usually lasted one to several minutes, but the last one observed by Camus and Vincent as they left the area began at 1511 and continued until 1525. Most of the eruption columns were dark, containing abundant ash but few blocks and no incandescent material. Water vapor could be seen condensing at the top of several eruption columns and lightning was occasionally observed. Ash fell on Sertung Island, about 2 km W of Anak Krakatau.

Siswowidjoyo, S., 1985, The renewed activity of Krakatau volcano after its catastrophic eruption in 1883: Proceedings of the Symposium on 100 Years Development of Krakatau and its Surroundings, Indonesian Institute of Sciences, Jakarta, p. 192-198.

Sudradjat, A., 1985, The morphological development of Krakatau volcano, Sunda Strait, Indonesia: Proceedings of the Symposium on 100 Years Development of Krakatau and its Surroundings, Indonesian Institute of Sciences, Jakarta, p. 141-146.

An increase in the volume of white fume from the summit crater was observed beginning 11 February. Similar activity, stronger than it had been for several years, continued intermittently until 28 February, when plume color darkened and emissions became more frequent. Plumes apparently rose a few hundred meters above the crater. On 1 March, glow was visible from the Java coast, . . . suggesting the presence of new lava. Earthquakes were felt 5-6 March from Anyer, 55 km E of the volcano.

On 16 March, a group from VSI visited Anak Krakatau. The eruption . . . had created a new crater on the SSE flank of the 1960-81 Vulcanian cone. The new crater was ~80 m wide and 150-200 m in length, with the long axis oriented approximately NNW-SSE. Its low point was on the SSE side at an altitude of 140 m. Lava emerging from a vent at that point fed one small flow moving SSW, and a second, longer flow to the SE. The second flow had descended from the 1960-81 cone onto the old crater floor (between the pre-1960 crater rim and the younger inner cone).

A single vigorous fumarole, on the SSW slope of the 1960-81 cone ~50-80 m W of the new 1988 crater, was active about every 5 minutes . . . . Reports from the forestry observer on Sertung Island (4 km W) indicated that incandescent blocks were ejected from this vent. Some of the blocks were thrown into the sea outside the new crater, ~500 m from the vent. During the 16 March visit, this vent was only mildly active, so geologists were able to enter it and collect samples from the current activity. Hand-specimen petrography indicated that the 1988 lava resembles the 1980 plagioclase andesite (SiO2 53-54%) and is clearly not like the dacite bombs of the 1981 Vulcanian eruption (SiO2 62-63%).

The eruption continued through April. During the first half of April, explosions from the 1988 crater occurred at the rate of 105/day; during the second half of April, the frequency decreased to ~45/day. Plume heights ranged from 100 to 600 m. A seismometer 800 m from the vent recorded neither deep nor shallow earthquakes during April; explosions and rockfalls may have saturated the instrument. Geologists have not visited the island since 16 March.

An eruption that began at 1802 on 7 November ejected lava fragments to 150 m height, followed by an ash explosion to 800 m. Increased seismicity during the first week in November preceded the eruption. A VSI team climbed the volcano on 12 November and reported that the eruption was from Anak Krakatau's NE crater. Lava flows extended 300 m NE and 100 m SE, filling a valley. Approximately 36,000 m2 of the island has been covered by an estimated 178,000 m3 of lava, mostly basaltic andesite with porphyritic to vitrophyric texture. Degassing and ejection of lava fragments was continuing on 12 November at about 3-minute intervals, to heights of 100-200 m. The number of explosion earthquakes decreased from 499 on 11 November to 406 on the 12th, and 296 on the 13th (figure 3). Volcanic tremor with a maximum amplitude of 30.5 mm and a frequency of 0.4 Hz was recorded from 1905 on 12 November until 0800 the next day.

Figure 3. Number of earthquakes recorded at Anak Krakatau, 1-13 Nov 1992. Courtesy of VSI.

Eruptive and seismic activity was continuing on 14 November. Based on the number of explosion earthquakes and the characteristics of volcanic tremor and occasional A-type events, VSI believes that the eruption may continue for several months at the current level of activity. VSI is discouraging visits to the island until further notice.

The eruption . . . was continuing in early December. Incandescent lava was ejected to 100-150 m height, with ash from intermittent explosions (at intervals of 3-32 seconds) rising 400-500 m. Explosion earthquakes remained frequent at 1,000-4,000/day, but no A- or B-type earthquakes have been recorded since 11 November. Lava flowed SE, and down the NE flank to the sea. Lava volume was ~5.5 x 106 m3, covering an area of ~2 x 106 m2. Tourists were advised not to visit the island until further notice.

The eruption . . . continued in 1993. The strongest explosive activity occurred on 12 November 1992. Bombs fell to several hundred meters N of the vent and smaller tephra reached the N coast. Lava flowed 1 km to the N coast and entered the sea, extending >100 m beyond the shore. Lava continued to advance in January, but feeding of the flow from the vent may have stopped by mid-February. Strombolian explosions ejected lava fragments, visibly incandescent at night, in early February and ash columns rose 100-400 m. The number of explosion earthquakes varied from 500-2,000/day (figure 4), at intervals of 5 seconds to 5 minutes. Explosions can sometimes be observed from the volcano observatory . . . . Tourists have been advised to remain at least 3 km from the island until further notice.

Figure 4. Number of daily explosion earthquakes, 10 November 1992 to 7 February 1993. Courtesy of VSI.

Activity has continued . . . since . . . 7 November 1992. A mid-November lava flow reached the NW coast and entered the ocean (figure 5). This lava flow continued to advance until early February. Beginning in February, a new lava flow traveled SSE, destroying seismic stations maintained by VSI and Gadjah Mada Univ (GMU). Another lava flow descended to the N in April, overflowed the old crater rim in May, and burned a forested area near the coast. Analyses of the April 1993 lavas showed 53.5% SiO2, an increase from the November 1992 lavas (51.3%).

Scientists from GMU reported degassing events and lava-block ejections on 6 May. Sharp thunder-claps were also heard, and ash fell heavily during most of their 8-hour visit. As of 13 June, the most recent activity observed by VSI geologists was a vertical Vulcanian explosion with a column ~150 m in diameter and 200-600 m high. There was no evidence of pyroclastic flows. Tephra deposits have built a new cone to at least 280 m asl, much higher than the previous peak (199 m elevation). A moderate explosion on 13 June killed one tourist and injured five others climbing on the old crater rim. Since the eruption began, tourists have been advised by VSI and the local government to remain at least 3 km from the island.

A new vertical seismometer was installed by VSI on 17 April ~10 m from the coast and 15 m asl on the SE side of the island (figure 5). Data are telemetered to the volcano observatory in Pasauran . . . . From 18 April to 7 May there were 4 days with >1,500 explosions and another 7 days with >1,000 (figure 6). Seismically detected explosions then gradually declined into early June. The number of explosion earthquakes/day was generally lower during this period than in late 1992 and early 1993.

Figure 6. Number of daily explosion earthquakes at Anak Krakatau, 18 April to 13 June 1993. Courtesy of VSI.

GMU placed a new seismic station on the E side of the outer crater rim on 6 May. Long-period and short-period 3-component measurements were also carried out during the visit to the island. Approximately events were detected during 6 hours of long-period seismic measurements (cutoff frequency 0.2 Hz). About 80% of these events could be correlated with visual observations of activity. Felt or observed earthquakes occurred an average of every 4 minutes, with stronger events every 7-8 minutes. Analysis indicates that the non-correlated seismic events had deeper hypocenters.

Anak Krakatau has now been in continuous eruption for nine months . . . . Continued explosions (140-400/day) have built a new cone to ~280-300 m elev, which is 80-100 m above the old peak (figure 7). A seismometer installed by VSI was destroyed by a ballistic bomb on 16 July; lava flows have previously destroyed two other seismometers. Earlier this year, lava flows filled the valley between the cone and the old rim, and overflowed onto the outer slope of the old rim. The April 1993 lava flow descended the N flank to the coast, where it burned trees. The recommended off-limits zone, 3 km from the island, remains in effect.

Figure 7. Sketch of Anak Krakatau showing topographical changes, as of July 1993, since the eruption began in Nov 1992. View is looking SE; scale is approximate. Drawn by Igan S. Sutawidjaja; courtesy of VSI.

In mid-August GMU scientists repaired damaged seismic equipment and conducted a seismic study on Anak Krakatau. GMU's seismic station was damaged by volcanic bombs on 18 May, only 12 days after installation. The new station consists of a 1-Hz vertical-component seismometer on the E flank of the island, closer to the coast and farther from the source vent than the damaged station (figure 5).

On 14 August the GMU team also deployed a 3-component seismograph with a 0.2 Hz cutoff frequency and collected data from 0900 to 1700. During this 8-hour interval >100 events were registered, with >90% correlated with minor explosions seen at the surface. In contrast to the bulk of the events, which had shallow sources, the events without visual correlation caused particle motions implying generation at greater depth . . . . Volcanically quiet intervals indicated little seismic contribution from ocean waves; such waves were chiefly of low amplitude and confined to the 0.5-3 Hz frequency range.

About four hours of the vertical-component seismic record are shown on figure 8. Many events appeared at 3-minute intervals. Longer intervals of quiet also occurred, and typically terminated in strong seismic shocks and eruptions. A more detailed 3-component record of a comparatively large event on 14 August (figure 9) shows relative quiet prior to the event, and near 0.2, 0.5, and 1.1 minutes, peaks in the seismic signal. The lower portion of figure 9 shows the computed smoothed spectra (from maximum entropy spectral analysis) for the three components.

During installation of the new system, scientists witnessed degassing and ejection of silica-rich volcanic bombs. They found no pumice. Observers on [Carita Beach] noted that lava glowed strongly in early May but had stopped by mid-June. As of 14 August glowing had not reappeared. [News reports during the 1994 eruption indicated that activity ceased in October 1993.]

Press reports have described renewed activity . . . from late March through early May. Activity apparently began again on 19 March after about five months of quiet. At the end of March, thick black ash plumes rose 300 m while "red flames and glowing lava" were observed at night to rise 200 m. Eruption noises like thunder could be heard at Carita Beach . . . . Scientists from GMU stated in the newspaper Suara Pembaruan that their seismograph was not operational at the time of the eruption, but began functioning again on 26 March.

Wimpy Tjetjep, Director of VSI, reported to the same newspaper on 6 April that volcanic materials had been thrown within a 3,000-m-radius from the crater. Ash and tephra emissions were continuing, and 160-170 weak volcanic earthquakes had been recorded in the previous week. Officials at the local observatory reportedly described the activity as "spouts of fire" and thick black ash emissions alternating with explosions. Ejection of blocks as large as 1-m-diameter were also reported. Fishermen at Carita Beach described the April activity as "gigantic fireworks floating in the middle of the sea at night."

The most recent report of activity was on 5 May, when VSI volcanologists at the local observatory told the Antara News Agency that an eruption ejected lava 200 m into the air. They also noted that as many as 222 "eruptions" had been recorded on 3 May. All of the reports emphasized that the island remains off-limits to visitors.

Ash explosions continued at a rate of 300-450/day in early August. The height of the ash columns, measured from the [Pasuaran Observatory] during clear weather, ranged from 150 to 500 m above the summit, with incandescent projections evident at night. The sporadic eruptions have deposited ash over almost the entire island. During the second week of August, explosion earthquakes averaged 460 events/day. Occasionally, explosion sounds were heard and vibrations felt at the observatory.

A pilot on a 16 October Qantas Airlines flight enroute from Perth to Singapore reported a light-gray, moderately dense, mushroom-shaped ash cloud rising to ~2 km altitude and drifting ~55 km W from the summit. Another pilot report on 28 November noted ash from the volcano rising to ~3 km altitude. A similar report on 18 December stated that an ash cloud was as high as 3 km and drifting E.

On 26 July 1994, J. Sesiano made observations from the sea, from the summit of the 1930 peak, and from the slopes of the active cone. Activity consisted of violent explosions (averaging one every 15 minutes) with ejecta (small tephra and bombs) erupted vertically to heights of 100-200 m that fell back into the crater. Finer particles were blown NW by winds. Longer intervals between eruptions (30-45 minutes) were followed by a single or series of very strong explosions that generated a dark plume 1,000-1,500 m above the crater. Ejecta from these stronger explosions, including some bombs >1 m3 in size, fell over the entire cone. Boulders were seen rolling down the slopes, reaching and slowly burying the November 1992-Apr 1993 lava flow (18:07), which had a surface temperature of ~30°C and was still degassing.

A rough examination was done on two thin sections made from fresh lava samples (collected while still hot). Phenocrysts were mostly zoned plagioclase, some augite, and rare olivine with reaction rims. The groundmass consisted of fine-grained opaque grains and scattered plagioclase, but no glass. Plagioclase composition, determined from very few properly oriented crystals, was An$45-55. Based on these observations, the samples were identified as andesite to basaltic andesite.

Volcanic activity continued through January-March 1995, sending grayish white plumes 150-500 m above the summit. Sounds like thunder were sometimes heard at the VSI observatory . . . and glow was visible at night as high as 50 m above the summit. The daily number of explosions in January and early February fluctuated between 50 and 150 events. From mid-February to mid-March the average number of explosions increased to 150-200 events/day (figure 10).

Figure 10. Daily number of explosion earthquakes (bars) and height of the ash plume (line) at Krakatau, January-March 1995. Courtesy of VSI.

According to news reports at the end of May 1995, authorities closed the volcano to tourists, permitting them to come no closer than 3 km. A VSI official told UPI that ~7,200 explosions were recorded during May; during the second week in June, ~2,000 explosions were recorded. Occurring every 3 minutes, the explosions shot ash ~150-400 m high.

The following supplements reports in 19:4, and adds information about April-June 1994 (VSI, 1994a). During March 1994 Strombolian eruptions had plumes that rose 50-400 m. These eruptions spewed incandescent ejecta every 5-10 minutes and were accompanied by sounds like "thunder-claps." From 26 March to the end of the month, 109-230 earthquakes were recorded each day. Similar Strombolian eruptions continued from April through June 1994, with the plume rising 50-300 m above the crater (VSI, 1994b). Incandescent volcanic materials were ejected to heights of 50-150 m above crater rim. Between 1 April and 17 May 1994, 50-450 earthquakes occurred each day. Following 30 days with an inoperable seismograph (16-30 June 1994), 10-600 earthquakes were recorded/day.

During June there were unofficial reports of unusually loud noises heard on the W coast of Java . . . . On 27 June, GMU scientists visited islands around Anak Krakatau and heard some very loud sounds; only some of which correlated to visual activity at Anak Krakatau. The observers compared their observations to a 1993 visit, when the volcano emitted steam-bearing discharges accompanied by lightning. The eruptions on 27 June 1995 appeared dissimilar because they were ash-rich and without visible steam. In addition, the 27 June eruptions produced string-shaped columns with mushroom-shaped tops; lightning was absent.

The group deployed two seismometers for five hours of observation. A vertical-component long-period seismometer (0.2 Hz cutoff) was put on Panjang Island, 3.6 km W of Anak Krakatau's summit, and a 3-component broadband seismometer was put on Sertung Island, 3.2 km WNW.

Typical seismograms, showing two of the three components recorded on Sertung Island, appear on figure 11. In one case, a low-frequency seismic signal arrived ~8 seconds prior to a sharp onset, reaching amplitudes of 0.6 mm/sec in the vertical component (figure 11). The second seismometer also recorded ~8 seconds of seismic signal before the onset of the air wave. Other events also showed the same 8-second delay between the seismic signal and these air waves. The case shown was correlated with a small eruption that generated a loud sound and ultimately spawned an ash cloud of undisclosed dimension. Assuming a shallow source for the eruption, the travel times for first arrivals of the strong impulsive signals across the 3.2 km source-to-receiver distance were on the order of 10 seconds, roughly the velocity of sound in air (0.33 km/sec). Thus, the strong impulsive signals were probably due to pressure waves transmitted through the air.

Figure 11. Seismic record from Anak Krakatau showing the vertical component for a typical event received on nearby Sertung Island. The data were collected on 27 June 1995 (t = 0.0 sec corresponds to 1049 local time). Approximate arrivals of the seismic and air-wave signals are indicated. Courtesy of Wahyudi and A. Brodscholl.

During an approved visit on 6 November, the volcano was steaming but not erupting. A large sulfur-stained plug of lava, ~20% the diameter of the summit cone, was bulging out of a black cinder cone just below and W of the summit; it appeared to be inside the SW margin of a broad depression. A smaller sulfur-stained plug was farther S in another depression. The landing site on the SE shore was a black-sand beach with tiny dunes of white pumice. While climbing the SE slope of the older cone, the party crossed water-eroded fields of pyroclastic material dotted with volcanic bombs. The ascent to the summit went through deep cinder deposits covered with a blanket of loose breadloaf-sized stones. From the summit complicated internal crater structures could be seen. One symmetrical cone (~100 m across) had a sulfur-lined inner rim that was fuming. The largest and most active fumaroles were inside this cone's S rim. A smaller cone within the larger one was almost horseshoe-shaped and steeper to the S. Bombs on the summit cone were as large as 1-2 m in diameter.

At about 1140 on 29 September, a Qantas Airlines pilot reported a thick plume near Krakatau that rose to an altitude of 3,700 m and drifted NW at low levels and E at high levels. There was no definite signature on GMS satellite images.

Although a pilot report described a 3.7 km tall eruption column from Anak Krakatau on 29 September (BGVN 21:09), ash columns during that same month more typically reached only 800 m above the summit. During the bulk of September explosions took place at 5-minute to two-hour intervals; bombs up to 20 cm in diameter reached the N and NE coastlines, areas lying ~1-1.5 km from the vent. Lava flows during July-August reached the island's W coast and added to its size. Two vents emitted lava and Strombolian eruptions in the N part of the main crater.

During October, ash explosions occurred every minute, followed by rumbling sounds and lava fountains as high as 600 m above the crater. The main crater produced all the activity during October with the other two craters remaining quiet. There were no lava flows released to the surface during October. However, weak red glow was occasionally observed at night (from the Pasuaran observatory).

The following describes the volcanism during March-May based on reports by the NOAA Satellite Analysis Branch (SAB), a team of the Société Volcanologique Européenne (SVE), and Mike Lyvers. Lyvers noted that the Indonesian government's 5-km exclusion zone around the island has not deterred local boat operators from anchoring offshore or even landing tourists on Anak Krakatau.

SAB reported that on 6 March at 0442 an unidentified aviator saw a significant eruption with ash reaching an altitude of ~7 km. This cloud, however, was not seen in GMS satellite imagery.

Members of the SVE visited the island twice in April. They learned that during March at Carita, a beach resort on the W coast of Java 40 km from the volcano, there were ashfalls and explosions from the volcano were heard. During April, emissions became less prominent and more irregular. During their first visit on 9-10 April they did not observe any plumes. After landing they ascended to the first crest line where the group encountered several bread-crust bombs and their substantial impact craters. As they were ascending the cone of the volcano the visitors felt the heated ground through their hiking boots. There were fumaroles on both the flank and the summit. The crater, 150-200 m in diameter, was breached to the W; the crater floor was occupied by large blocks, and it was possible to distinguish two vents aligned on a fissure trending SE-NW.

The group returned on 17-18 April, after another eruptive episode. This time they observed enormous new blocks at the summit. The S vent continuously emitted white steam; the N vent sporadically discharged brown-black ash that rose up to 500 m above the vent. The SVE team watched from a spot in front of the cone, ~400 m from the summit, when at 1820 the S vent exploded generating an ash plume and throwing incandescent projectiles ~200 m above the crater. One projectile landed very close to the observation point. The next morning, ash on the tents suggested that the volcano had another explosion. The group witnessed another eruption as they were leaving the island by boat at 1000.

SVE members learned that after spending 21-22 April on the island, Guy de St. Cyr (a French tourist-guide) saw plumes accompanied by projectiles. He described the ash as an unusual pink color. During the night, incandescent explosions were took place about every 30 minutes; several incandescent blocks fell over the dome's N side. The next morning, during a boat tour around the island, some blue smoke rose from mid-way up the W-SW flanks of the dome, conceivably a sign of minor lava flows.

During the afternoon and evening of 17 May, Mike Lyvers visited the island by boat. The previous few days, when observed from Carita Beach, the volcano had been quiet. In contrast, on 17 May it erupted almost continuously, issuing minor amounts of ash and sometimes a few bombs. Occasionally, larger explosions sent incandescent ash high into the sky, generating a spectacular display of volcanic lightning and covering the cone with glowing bombs. The volcano seemed to show no obvious pattern to its activity, with random fluctuations in the intensity of eruption.

Krakatau erupted on 5 February 1999 accompanied by thunderclaps and an ash plume that reached a height of ~1,000 m above the summit. The activity continued until 10 February with ash plumes reaching ~100-300 m above the summit. The continuing sporadic eruptions deposited small amounts of ash over most of the island; a deposit of ~0.3 mm was measured near the observatory. On 11 February, the glow of ejecta was observed reaching ~25 m above the summit and continued during the night.

Activity decreased early during the week of 9-15 March. Weak booming noises were heard twice on 9 and 10 March, but plumes were not observed. At the end of the week booming noises were rare, and a white-gray ash plume was seen on 14 March that rose 100-300 m above the summit. The current activity is a continuation of eruptions that began in 1992.

After a repose of twenty months Anak Krakatau erupted again at 1615 on 5 February (BGVN 24:02). Several scientists, including some from the Volcanological Survey of Indonesia (VSI) and from Unocal Geothermal of Indonesia, visited Krakatau from 28 March to 6 April. This report combines their observations.

Seismic activity preceding and coincident with the eruption went undetected because of ballistic bomb damage to seismometers. Until 3 April, activity typically involved 5-10 explosions per day. Beginning at about 1500 on 3 April ash explosions became almost continuous (figures 12 and 13). During the interval 0955-1230 on 4 April, the volcano erupted every 1-3 minutes from a new crater a few hundreds of meters S of the summit crater that formed during 1992-97. Accidental blocks, lava bombs, and ash reached heights of 250-300 m above the crater rim. About a third of the eruptions were Strombolian, with showers of lava and bombs (occasionally 1 m across) ejected 50-100 m above the vent and falling onto the upper flanks. Some ballistic fragments 20-30 cm in diameter rose above the associated ash cloud and landed 800 m from the vent on the upper flanks before rolling down to the shore. Eruptions were often accompanied by thunderous blasts and rumbling sounds heard several kilometers from the crater, including at Pasauran and Kalianda observatories 42 km from Krakatau.

Figure 12. Lava and bombs exploded from the summit of Anak Krakatau on 4 April. This view was from the sea looking toward the E. Courtesy of VSI; photo by Karsten Moran, Jakarta International School.

Figure 13. Ballistic fragments flew above and to the side of rising ash clouds during the eruption at Krakatau. The view is toward the N. Courtesy of VSI; photo by Karsten Moran, Jakarta International School.

A wedge-shaped deposit of fresh ash and bombs was visible on the crater rim (the rim is higher on the SE due to prevailing northwesterly winds that blow ash and other ejecta in that direction). Ash clouds were light gray. Observers noticed fine black ash that fell on their boat as they passed under the plumes ~500 m downwind from the crater. The ash was crystal-poor and frothy, suggesting that it was mostly juvenile material.

A solfataric plume originating at ~200 m elevation on the N flank discharged steam and bluish gas. Nearly a dozen other solfataras discharged steam and non-condensible gas and deposited bright yellow native sulfur around vents near the summit (figure 14). Another fumarolic area was centered at 140 m elevation on the W flank below the active crater.

Figure 14. An ash plume rises from the summit crater above a fumarolic area on Krakatau's W flank, seen here looking toward the NE. The light-colored patches are mostly native sulfur. Courtesy of VSI; photo by Karsten Moran, Jakarta International School.

Scientists observed several boatloads of tourists who had landed on the accessible SE beach. Officials had closed an area of 3 km radius around the vent, but many tourists defied the prohibition and climbed to the ridge 400 m from the summit vents. Escaping gases continued to pose a very serious hazard.

The renowned Krakatau volcano lies in the Sunda Strait between Java and Sumatra. Caldera collapse, perhaps in 416 AD, destroyed the ancestral Krakatau edifice, forming a 7-km-wide caldera. Remnants of this volcano formed Verlaten and Lang Islands; subsequently Rakata, Danan and Perbuwatan volcanoes were formed, coalescing to create the pre-1883 Krakatau Island. Caldera collapse during the catastrophic 1883 eruption destroyed Danan, and Perbuwatan volcanoes, and left only a remnant of Rakata volcano. The post-collapse cone of Anak Krakatau (Child of Krakatau), constructed within the 1883 caldera at a point between the former cones of Danan and Perbuwatan, has been the site of frequent eruptions since 1927.

During six lava-producing eruptions between 1958 and 1980, flows moved S and SW from the SW crater. Observations are frequently made from Carita Beach on the coast of Java, ~40 km E. The local VSI volcano observatory is at Pasuaran, ~42 km E.

Following several months of intense activity that began on 5 February (BGVN 24:04), Anak Krakatau became relatively quiet in late April. From the end of April until the end of May, only several explosions were heard. On 26 April a weak explosion sent a white-gray ash plume 200-500 m high. Between 4 and 17 May there were two blasts per week, each accompanied by a glow and white-gray ash reaching between 100 and 400 m high. In the week from 18 to 24 May, in addition to two explosions, a shock on the morning of 20 May registered at 2 on the MMI scale.

Anak Krakatau was very active from 1992 to 1997, depositing 6.8 x 106 m3 of lava flows. The island was enlarged by 378,527 m2 and the height of the cone increased from 159 to ~300 m.

The Strombolian eruption of Anak Krakatau continued during May-August. White-gray ash was emitted in plumes up to 600 m above the summit. Detonations were accompanied by the ejection of incandescent material. A new seismograph was installed on 27 May, replacing one destroyed by falling lava bombs in April (BGVN 24:05). During many weeks of May and June thousands of explosions were recorded on seismographs. An earthquake of intensity MM II was recorded on 10 June and another of the same intensity occurred during the week ending 16 August. On several days during the reporting period the volcano could not be seen because of thick haze.

This report covers the period of 29 May-26 June 2000. Krakatau volcano was last reported on in BGVN 24:08 (August 1999) and had remained at a stable level of activity up until 0600 on 29 May, a time when seismometers began registering ~150 earthquakes attributed to the beginning of an eruption. Cloud cover made visual inspection impossible on 29 May but inspection on 30 May revealed a dark gray ash plume rising 500 m into the air. Further seismic data from 30 May registered an increase in the number of both deep and shallow volcanic earthquakes; eruption and emission earthquakes increased to ~330 events daily before dropping back down to ~200 events on 31 May. During 3-5 June seismicity remained elevated. A white plume reached ~20 m on 3 June, a very dark plume rose to ~50-500 m on 4 June, and a gray plume ascended to ~50-300 m on June 5. Bad weather inhibited visual observation throughout the rest of the reporting period.

Three booming explosions on 17 June coincided with a decrease in the number of volcanic earthquakes, as well as a marked increase in small explosion earthquakes. Seismic data for the week of 20-26 June contained several large and small explosion earthquakes; a thick gray ash plume was emitted on 25 June, rising to ~250 m. Krakatau remained at alert level 2 (on a scale of 4).

Krakatau activity continued after the previous report (BGVN 25:05) through October 2000 although intensity decreased relative to the 29 May 2000 eruption. The volcano's hazard status, however, did not exceed 2 (on a scale of 1-4) within the report period. During 27 June-2 July explosions sent ash to heights up to 500 m, and booming sounds could be heard on three occasions. VSI reports resumed as of 25-30 July when seismographs recorded 1,961 explosion earthquakes, compared to 441 about a month earlier. An infrasonic sensor detected 37 events. A volcanic ash advisory was issued based on a pilot report of ash observed at an altitude of ~6,100 m. Satellite imagery did not detect a significant plume on this date, and no additional ash advisories were dispatched.

Activity remained similar through 15-21 August. Frequent booming was heard, and high numbers of explosion and infrasonic earthquakes were detected. A volcanic ash advisory was issued on 20 August although it indicated that plumes were sparse, did not reach high altitudes, and dissipated quickly. During 22 August-4 September a white, low-density plume rose 50 m above the summit. No visual observations could be made due to heavy fog, clouds, or smog masking the summit from view during 5 September-30 October, although seismicity indicated persistent activity. During 5-18 September, 3,220 explosions and 17 infrasonic events were recorded. Audible booming, however, ceased on 12 September, and activity decreased dramatically through the end of October. Deep volcanic (A-type) earthquakes stopped occurring as of 10 October, although a low number of small explosion earthquakes and tectonic earthquakes continued through 30 October. No further VSI reports were issued for Krakatau in 2000.

Eruptive activity at Krakatau through late October 2000 was described in BGVN 26:01. The Volcanological Survey of Indonesia (VSI) did not report any further activity until mid-March 2001, when the number of shallow volcanic (B-type) earthquakes rose to 79 from 25 the previous week. The number of shallow volcanic earthquakes decreased again in late March to 34. In early April, seismic activity at Krakatau increased again. The seismographs detected 7 deep volcanic (A-type) earthquakes, 54 shallow volcanic earthquakes, and 7 tectonic events.

Local tour operators reported a significant increase in seismic activity at Krakatau beginning during July 2001 and continuing through August. During 9-15 July there were 728 shallow volcanic earthquakes registered. [On 21 July an explosion occurred accompanied by a booming sound. The explosion was recorded by an infrasonic microphone sensor installed at the Pasuaran post observatory.] At the beginning of August the volcano emitted ash to 500 m and ejected ballistics onto the flanks of the main cone.

John Seach visited on 12 August and found that the volcano was not erupting then, but was steaming vigorously on the N side of the summit crater. Pulses of steam every minute reached 20 m above the summit. Lava bombs, 0.5 m in diameter, littered the old crater rim (840 m SE of the of the summit at 167 m elevation) at a density of 1 /m2. The bombs left 1.5-m-diameter impact craters up to 1.5 m in diameter at distances of 1.3 km SE of the summit. The fresh impact craters were caused by both lithic and lava bombs. Observers on a boat 1.5 km W of the volcano noticed a strong sulfur smell. The top 150 m of the active cone was steaming from multiple locations. On the NW side, 60 m below the summit, a fumarole emitted blue gas.

During 3-9 September the number of explosion and volcanic earthquakes increased, but the number of small explosion earthquakes sharply decreased 12-18 September. Krakatau remained at Alert Level 2 (on a scale of 1-4).

A thick white plume reached 25 m above the summit several times during October through December 2001. During 27 August 2001 through 16 September 2001 at Krakatau, available seismic data were dominated by explosions and shallow volcanic earthquakes (table 1). The seismograph broke on 16 September 2001 but was repaired by 26 August 2002, when it showed a slight increase over the previous interval when data were available. No surface activity accompanied the increased seismicity. Volcanic events decreased during early September. The volcano remained at Alert Level 2 through at least 8 September.

Table 1. Earthquakes registered at Krakatau during 27 August 2001 through 8 September 2002. The seismic system was down during 16 September 2001-25 August 2002. Courtesy of VSI.

During 9 September through at least late December 2002, seismicity at Krakatau was dominated by A-and B-type volcanic earthquakes (table 2). Throughout the report period, clouds obscured the view of the summit. Krakatau remained at Alert Level 2.

Table 2. Earthquakes registered at Krakatau during 9 September-29 December 2002. No data were available during 16-29 September. Courtesy VSI.

Throughout 2001 and 2002, MODIS thermal alerts for Krakatau occurred only during July-September 2001. The first alert occurred on 31 July when one alert pixel was detected with an alert ratio of -0.793. The anomalies increased during August and on 9 August the anomaly consisted of two alert-pixels with a maximum alert ratio of -0.306. Other major anomalies occurred on 1 September (four alert-pixels with maximum alert ratio of -0.327) and on 17 September (two alert-pixels with maximum alert ratio of -0.284). These anomalies correspond to an increase of activity at Krakatau characterized by ash and bomb emission during August 2001 and an increase in the number of explosion and volcanic earthquakes during the first half of September 2001, reported by the Volcanological Survey of Indonesia (BGVN 26:09 and 27:09). The coordinates of the centers of the alert pixels are tightly grouped around the summit of the main cone. Bearing in mind that each pixel represents radiance from an area of ground more than 1 km across, the alert pixels could represent radiance from the active vent or from hot ejecta close to the vent.

According to reports from the Volcanological Survey of Indonesia (VSI), no visual observations were made this month due to foggy weather. The volcano remained at alert level 2 for the month. They also noted that relatively few volcanic and tectonic earthquakes were recorded during the weeks of 2-8 and 9-15 June 2003. Specifically, the 2-8 June record consisted of 9 deep volcanic earthquakes, 19 shallow volcanic earthquakes, and 5 tectonic earthquakes; the record of 9-15 June consisted of 6 deep volcanic earthquakes, 17 shallow volcanic earthquakes, and 4 tectonic earthquakes.

In the week of 16-22 June, a significant increase in shallow volcanic earthquakes was observed, although no tectonic earthquakes were recorded. The sesimic record for that week showed 11 deep volcanic earthquakes and 63 shallow volcanic earthquakes. Both volcanic and tectonic earthquakes were recorded for the week of 23-29 June, with 7 deep volcanic earthquakes, 61 shallow volcanic earthquakes, and 2 tectonic earthquakes detected.

Due to continued foggy weather, no visual observations could be made at Krakatau during July and through 17 August. Throughout this period the volcano remained at Alert Level 2. Seismicity reported by the Volcanological Survey of Indonesia (VSI) between 30 June and 17 August consisted mostly of shallow volcanic events (table 4), although 36 deep volcanic earthquakes were recorded during the week of 30 June-6 July.

A report of activity at Krakatau for the period 18-24 August was provided by the Volcanological Survey of Indonesia. There was increase in volcanic earthquakes during this time, while tectonic earthquakes decreased. No visual observations were made due to foggy weather. Seismicity consisted of 12 deep volcanic earthquakes, 56 shallow volcanic earthquakes, and three tectonic events. The hazard status was at Alert Level 2 (on a scale of 1-4).

Intense activity occurred at Anak Krakatau beginning on 4 July 2004, when gas and steam emissions increased. The number of volcanic earthquakes also increased on 5 July to between 1 and 4 events per day, then abruptly rose to as high as 58 events/days during the week of 12-18 July before dropping again to 2-17 daily events (table 5). Based on the increased activity, the hazard status was upgraded to Alert Level II (yellow) on 16 July; visitors were not allowed to approach the summit or crater. Seismicity recorded at the Kalianda station after 18 July through 15 August was variable, but did not exhibit the high numbers recorded in the first half of July.

Eruptive activity in recent years was low at Krakatau. The Indonesian volcanological monitoring agency, now called the Center of Volcanology and Geological Hazard Mitigation (CVGHM), did not report any eruptive activity between June 2005 and September 2007. Seismic data collected during this period (figures 15 and 17), although intermittent and variable, suggests mainly low-level activity (discussed in more detail below).

Figure 15. Volcano-tectonic earthquakes recorded at Anak Krakatau, June 2005-early September 2007. Grayed-out areas represent periods when seismic data were not available. Courtesy of CVGHM.

Figure 16. Photograph of an ash plume from Anak Krakatau, 1 November 2007. View is to the SE from a monitoring station on Sertung island. Rakata island is in the background. Courtesy of CVGHM.

Figure 17. Satellite image of Anak Krakatau showing part of the monitoring network. Stations KM01, KM02, and KM03 are equipped with seismometers (broad-band at KM01) and GPS systems for deformation monitoring. A weather station is installed at KM01, a sea-level sensor at KM02. An electro-magnetic station (KM05) is located near station KM01. Gases are monitored at a nearby fumarole. Courtesy of CVGHM.

Starting on 23 October 2007 reports noted multiple gray plumes from eruptions lasting 3-6 minutes; these vented from a crater near the summit of Anak Krakatau (figure 16). The eruptions and associated increased seismicity during 23-26 October 2007 prompted CVGHM to raise the Alert Level to 3. Poor weather conditions allowed only intermittent observations, but plumes rose to an altitude of ~ 1 km during 23-26 and 30 October. Similar eruptions were continuing in early November (figure 16).

Activity during April 2005-September 2007. On 13 April 2005 increased seismicity prompted authorities to raise the Alert Level to 2 (on a scale of 1-4). Seismic activity decreased over the next four days to a normal level. Visitors were banned from the summit and crater of Anak Krakatau due to toxic gas emission. Another increase in seismic activity was reported around 16 May. Elevated seismicity was also recorded on 24 September 2005, 8 December 2005, and 18-19 June 2006 (figure 15).
On figure 15, a conspicuous, longer period of high seismicity occurred during most of December 2006, when tremor and low-frequency events also increased. That peak on figure 15 ended prior to the end of the month. No eruptions were noted in available reports by CVGHM for these episodes of elevated seismicity in 2005 or 2006. For the intervals where data were available during the first eight months of 2007, seismicity was generally moderate to low.

Monitoring. The monitoring system (KRAKMON) consists of a number of geophysical, gas-geochemical, and environmental measuring sites on the Krakatau island complex. All data are acquired continuously and are transmitted to the Pasauran Observatory (western Java) via digital radio telemetry. In Pasauran, the data are collected and transmitted to a server in Jakarta. From there, the data were accessible through internet (http://krakmon.vsi.esdm.go.id/). Three stations are located on Anak Krakatau (figure 17). A fourth station on Sertung island consists of a short-period seismometer and a digital camera with a view of Anak Krakatau.

During 23-26 October 2007, minor eruptions occurred at Anak Krakatau (BGVN 32:09), an island and active vent on the rim of the famous larger caldera whose name often is misspelled as "Krakatoa." This report continues coverage from late October through November 2007. The Center of Volcanology and Geological Hazard Mitigation (CVGHM) raised the Alert Level to 3 (on a scale of 1-4) for Krakatau on 26 October because of the presence of multiple gray plumes from the volcano and an increase in seismicity. Plumes rose to an altitude of ~ 1 km during 23-26 and 30 October. Villagers and tourists were advised not go within 3 km of the summit.

As reported by VolcanoDiscovery's Tom Pfeiffer, who visited there from 21-26 November, emissions were relatively constant. He noted that all activity occurred from the newly formed crater on the upper S flank just below the old summit crater (figure 18). On 21 November, the new crater had an oval shape, approximately 50 x 70 m. Dense, dark brown, billowing ash clouds escaped in pulses from the crater at near-constant intervals of about 2 minutes, rising typically 100-200 m above the crater and drifting E. A few blocks were ejected along with the ash clouds (figure 19).

Figure 18. A sudden explosion ejecting rocks and ash on the S flank of the old Anak Krakatau crater on 22 November, 2007. Courtesy Tom Pfeiffer of Volcano Discovery.

Figure 19. Ballistic blocks land all over the cone of Anak Krakatau where the impacts stir up dust on 22 November 2007. A few also flew as far as the sea. Courtesy Tom Pfeiffer of Volcano Discovery.

Pfeiffer also reported that at more irregular intervals, about 10-30 min apart, more violent, small vulcanian explosions interrupted the weaker ash venting events. The more violent explosions consisted of a sudden spray of mostly solid rocks and few incandescent scoria, followed by more powerful and turbulent ash plumes that rose up to 1 km above the crater (figure 20). Generally, these vulcanian explosions occurred after a slightly longer quiet period and, in most cases, the length of the quiet period correlated with the force of the explosion.

Pfeiffer noted that several more powerful explosions occurred at intervals of approximately 16-24 hours. The strongest, on 21-22 November, showered the whole island with incandescent blocks, ignited bush fires, and produced a very loud cannon-shot noise that rattled windows on the W coast of Java, 40 km away (figure 21).

Figure 21. On the evening of 21 November 2007, a powerful blast throws bombs and blocks all over the old cone of Anak Krakatau. Courtesy Tom Pfeiffer of Volcano Discovery.

Other, unusually large blasts occurred at around 0200 on 21 November and at around 0900 and 1320 on 23 November (figure 22). Early on 23 November, activity became more ash-rich and the vigor of the individual events increased slightly over the next two days. The pace of single explosions stayed at near-constant intervals of about 2 minutes. During 24-25 November, ash plumes typically rose to over 1 km above the crater and were easily visible from the W coast of Java. Based on a pilot report, on 24 November, the Darwin Volcanic Ash Advisory Center noted that an ash plume rose to an altitude of 3 km and drifted NE.

Figure 22. Another very powerful blast occurs at around 0300 on 24 November 2007. Incandescent blocks reach the lower western flanks of the island. Courtesy Tom Pfeiffer of Volcano Discovery.

Based on the University of Hawaii's Institute of Geophysics and Planetology (HIGP) Thermal Alerts System MODVOLC analysis of MODIS (Moderate Resolution Imaging Spectroradiometer) satellite thermal anomaly data, occasional hot spots were identified by Terra or Aqua satellites. The thermal alerts occurred on twelve occasions between 27 October and 9 December 2007. Seven of these took place between 16 and 26 November 2007.

Renewed eruptive activity from Anak Krakatau began in October 2007 (BGVN 32:09), with minor eruptions through that November (BGVN 33:01). This small but growing post-caldera cone first gained attention with a 1927 eruption (Simkin and Fiske, 1983). During October-November 2007 several eruptions were Vulcanian in nature (BGVN 33:01). The detailed chronology of behavior during October 2007 to 3 July 2009 is sometimes sketchy, but activity was apparently quite variable. Although one or more lulls may have occurred, eruptions clearly continued into 2009.

Many of these eruptions were minor, but some were large enough to cause the Center of Volcanology and Geological Hazard Mitigation (CVGHM) to raise the Alert Level to 3 (on a scale with 4 as the highest). The Alert Level was lowered and raised again throughout 2008 and into 2009 as activity warranted. People were advised not to go within 1.5 km of the summit.

During April 2009 some residents in neighboring Sumatra allegedly evacuated when they saw more intense activity (including plumes up to ~ 1 km above the crater). Some of the taller plumes during the reporting interval rose to ~ 3 km.

Activity through August 2008. According to a news article, by 22 November 2007, seismicity had declined in frequency. Based on an Antara News article, this decline in seismic activity was interrupted by incandescent rock ejections on 20 January 2008 accompanied by plumes that rose to altitudes of 2.8-3.3 km. Eruptions reportedly had a "deafening sound" and could be seen from Sertung and Rakata islands. Seismicity again declined in early February 2008, and eruption plumes and ejected incandescent material were not seen during 4 February to mid-April 2008.

Seismicity increased during 14-21 April 2008, with the number of events per day peaking on 20 April. Ash plumes accompanied by ejected incandescent rocks were noted during CVGHM field observations on 16, 17, and 18 April. The eruption affected the summit and the E and S flanks. Booming noises were reported and occasionally heard at an observation post 42 km away.

Based on observations of satellite imagery and pilot reports, the Darwin Volcanic Ash Advisory Center (VAAC) reported that a low-level ash plume on 20 June 2008 rose to an altitude of 3 km and drifted NW.

During 22 June-1 July 2008, the number of seismic events decreased significantly and booming noises were less frequently heard. On 1-3 July ash emissions declined, although on 1 and 2 July low level ash plumes rose to an altitude less than 3 km and drifted NW.

Based on observations of satellite imagery, the Darwin VAAC reported that ash plumes from Anak Krakatau rose to an altitude of 3 km on 27 July 2008 and drifted NW.

According to an article in Antara News, eruptions increased in frequency during 10-11 August 2008. On 12 August, monitoring personnel reported that active lava flows and dense emissions of "smoke" continued, but that the frequency of earthquakes and eruptions had declined. Another news article indicated that explosions and earthquakes averaged ~ 120 per day during 11-17 August 2008. Monitoring personnel during that period observed plumes, active lava flows, and rock ejections.

Activity during March-May 2009. No additional reports by CVGHM were available during September 2008 through February 2009. Alerts based on thermal anomalies (see MODVOLC section, below) were not present during 31 August 2008 to 30 March 2009.

Seismicity increased significantly during 19-25 March 2009 and remained high through 5 May. During periods of clear weather on 25 March, white-to-gray plumes rose 400 m above the volcano. During 27-30 March and 1 April 2009 clear weather revealed ash plumes rising 200-800 m. On 2 April an ash eruption was seen on satellite imagery and reported by a pilot. A resultant ash plume drifted more than 60 km S.

During March through 25 April 2009, an episode of heightened seismicity produced thousands of eruptive signals (table 6); however, the seismic station shut down overnight during 1-26 April, and completely shut down during 27-29 April. CVGHM believed that this shutdown was the result of either blockage of sunlight from reaching the solar panels by tephra collecting there or because of impact-induced damage to the panels. On 29 April CVGHM installed a seismometer on Anak Krakatau at a location thought to be reasonably safe.

Table 6. Type and number of earthquakes and tremor recorded at Krakatau during 27 March-6 May 2009. Values shown are daily averages unless otherwise indicated by footnotes below. 1Average during12 hour period (daylight). 2 Starting at 0830 local time from a new, safer location. 3 During 0000 to 1200 local time. Courtesy of CVGHM.

During April 2009 observers reported grayish-white to black plumes that rose to 50-1,000 m above the crater. They heard many loud booms. CVGHM observations carried out on 24-25 and 29 April found the eruption venting from a crater near the volcano's peak on its SW slope. Eruptions generally sent incandescent blocks and ash ~ 500 m from the center in all directions. Some of the lofted ash blew E to SE and caused fallout up to 5 km away.

According to a news article on 29 April 2009, some residents in southern Sumatra near Krakatau evacuated because they had observed increased volcanism during the previous week. For example, observers reported loud blasts, lava flows, and ash plumes. In clear weather on 5 May "smoke" rose 500 m above the crater.

An Antara News article published on 18 June 2009 indicated that in the previous several days the number of small eruptions increased tremendously. It said that, according to Anto Prambudi, head of the monitoring post in Pasauran village, at least 828 small eruptions were recorded during 11-17 June 2009.

MODVOLC. MODVOLC thermal alerts were triggered through 9 December 2007 (BGVN 33:01). In later 2007, comparatively few alerts occurred, but became more prevalent again during mid-January 2008. After that, they were few or absent until mid-April; alerts were common and strong during the week ending 4 May. Consistent alerts were the pattern until the week ending 7 June, which had no alerts, but some continued in the next few weeks.

A seven-month gap in MODVOLC thermal alerts occurred during the interval 31 August 2008 to 30 March 2009. After that, alerts again became common again, particularly abundant during April 2009 (an episode of eruptions and heightened seismicity) and continued regularly through at least 3 July 2008.

The gap in alerts may have been influenced by downward biasing from poor weather conditions. On the other hand, for the cases with high numbers of alerts, false positives (due to fires for example) were unlikely on the desolate landscape of Anak Krakatau.

Our previous report on Krakatau (BGVN 35:05) covered activity from October 2007 to 18 June 2009, discussing an increase in seismicity and eruptions that began in late March 2009 after a 7-month gap in MODVOLC thermal alerts. This report chiefly focuses on activity during 3 July 2009 through 29 October 2009 based on Volcanic Ash Advisory Centre (VAAC) reports and monitoring from coastal Java at the Pasauran observation post in Banten province.

As an overview of hazard status during the past few years, the Center of Volcanology and Geological Hazard Mitigation (CVGHM) noted that starting 3 July 2008, Anak Krakatau had been at Alert Level 2 (Waspada). On 25 March 2009 activity increased significantly, with the recording of 19 eruptions. Eruptive activity then continued without pause until 6 May 2009 when the hazard status was elevated to Alert Level 3 (Siaga). Based on a decline in seismic and eruptive activity, on 31 October CVGHM decreased the status to Level 2.

The Darwin Volcanic Ash Advisory Center (VAAC) reported that a pilot observed an ash plume at 1948 (local time) on 3 July 2009 that rose to altitude below 3 km. The VAAC did not detected the plume on satellite imagery.

According to CVGHM, activity steadily declined during August and to at least as late as 29 October 2009 (table 7). Visual observations of the summit were often obstructed by fog, although a gray cloud rising as high as 1,300 m above the summit was observed at a clear moment on one undisclosed day in August. Booming eruptive sounds were recorded 49 times in August and once in September.

Table 7. Seismically detected signals at Krakatau, including tremor and earthquake, registered by the Pasauran observation post during August 2009 through 29 October 2009. Instrument locations were discussed in BGVN 32:09 (although the status of these stations was not discussed). Where reported the table also presents ranges of amplitudes (in millimeters, mm), duration (in seconds, sec.), and S-wave minus P-wave arrival times (S-P). Courtesy of CVGHM.

CVGHM also reported two major seismic events over the reporting period at distance from Krakatau with little discernable impact there. On 16 August 2009 at 1438, a tectonic earthquake originating on the southern side of Pulau Siberut island, located ~ 900 km to the NW of Krakatau, was recorded with a magnitude of 6.9. On 16 October 2009, a tectonic earthquake originating from Ujung Kulon-Jawa Barat, located at the W tip of Java, occurred with a magnitude of 6.4. CVGHM stated that the volcanic seismic signals at Krakatau following the earthquakes on 16 August and 16 October do not indicate an increase in volcanism.

Frequent MODVOLC thermal alerts were recorded during 1 April to 31 August 2009.

Our previous report on Krakatau (BGVN 34:11) was published at the end of the 2009 eruptive phase. Anak Krakatau (the active post-collapse cone of the infamous Krakatau volcano, figure 23) began another eruptive phase around 25 October 2010, characterized by up to hundreds of explosions per day (e.g., 251 explosions during 31 October-1 November 2010 as noted in the Jakarta Post). In November 2010, Arnold Binas documented explosive activity, ballistic bombs, column collapse generating pyroclastic flows, and volcanic lightning in some spectacular photographs (figure 24).

Figure 23. Topography and bathymetry of the Sunda Arc showing several noteworthy features including Krakatau. Inset shows the active Anak Krakatau, a comparatively small island. The star symbol shows the location of the Great Sumatran Earthquake of 2004. Adapted from Jaxybulatov and others (2011).

Figure 24. Photographs of explosive activity at Anak Krakatau during November 2010, within approximately one month of the onset of the eruptive phase beginning around 25 October 2010. (top) An explosive plume and column collapse generating a pyroclastic flow (note the material falling from the plume). (bottom) An explosion at night, capturing the parabolic arcs of red, glowing bombs thrown from the crater and the brilliant white tracks of lightning. Courtesy of Arnold Binas.

According to the Center for Volcanology and Geological Hazard Mitigation (CVGHM) and the Darwin Volcanic Ash Advisory Centre (VAAC), ash plumes reached heights of 2.4 km altitude on 24 December 2010 and 12 January 2011; on 15 January 2011, an ash plume reached 3 km altitude.

Ash fall in early January 2011 covered the solar panels running seismometers monitoring Anak Krakatau, rendering them inoperative until rainfall cleared the ash from the solar panels. Following the seismometer failure and amid continued activity of the volcano, the South Lampung Disaster Mitigation Agency (BPBD) ordered the evacuation of seven districts (of the 17 districts of South Lampung Regency) on 11 January 2011 (Jakarta Post).

In March 2011, the activity at Anak Krakatau decreased and the eruption was considered to be over. According to a local news service (TO:DAY), the head of the Anak Krakatau monitoring post reported that in the previous month, volcanic earthquakes became fewer and at greater focal depths. They had ranged from a high of 500-600 shallow earthquakes per day, declining to dozens per day in March 2011.

In August, Andi Rosadi reported on the Volcano Discovery website that the seismometer monitoring Anak Krakatau was again damaged by explosions venting from the summit crater, noting "many quite big new bombs around the old crater." CVGHM reported that GPS measurement stations had been damaged as well. Rosadi also described and photographed large areas of incandescence within the summit crater on 8 August 2011, reporting a lava dome or "dry lava lake glowing in the central crater" (figure 26). The Volcano Discovery website noted that on 6 September 2011, Chris Weber reported fire fountaining reaching ~10 m high.

Figure 26. Incandescence within the summit crater of Anak Krakatau seen from similar perspectives on 8 August 2011, at 1731 (top), and at 1830 (bottom). Courtesy of Andi Rosadi, Volcano Discovery.

On 30 September, CVGHM reported that the seismometers were again operational following 18 September, and increased the Alert Level from 2 to 3 (on a scale from 1 to 4), citing visual and seismic observations. Visually, while no ash producing eruptions had occurred from 14-30 September, CVGHM reported that the E flank of the volcano was covered in solfataras, with thin gusts of smoke within the crater and along crater walls. They also reported vibration in the body of the volcano within distances of 700 m from the summit. Seismically, from 18-30 September, CVGHM reported persistent volcanic earthquakes (volcanic earthquake swarms), noting that volcanic earthquakes had reached a level of 4-5 events per minute. As of 3 October, the Alert Level remained at 3, and residents and tourists were prohibited within 2 km of the crater.

Plumbing of the Krakatau system.Three recent publications addressed magma generation and plumbing of the Krakatau system from different disciplinary approaches. Employing mineral-melt equilibria thermo-barometry on Krakatau's phenocryst phases of clinopyroxene and plagioclase, Dahren and others (2010) demonstrated that clinopyroxene crystallizes at 8-12 km depth, while plagioclase crystallizes at 4-6 km depth. The two crystallization depth ranges could suggest multiple magma storage regions in the Krakatau system. They also noted that clinopyroxenes erupted before 1981 recorded deeper crystallization (and thus storage) - depths of 8-22 km - indicating a shallowing of the plumbing system over the last ~ 40 years.

From seismic tomography and inversion, Jaxybulatov and others (2011) reported a multi-layered structure beneath the Krakatau system (figure 27). They found their model similar to the one noted above (Dahren and others, 2010). They also reported that the deep-focus earthquakes they studied (100-160 km focal depths) were likely related to phase transitions and fluid fluxes in the subducting slab. They suggested these could lead to diapirism and formation of magma chambers in the overlying mantle wedge and crust.

In their spatial and temporal analysis of seismological data from 1964-2005, Špièák and others (2011) reported both deepening and shallowing of earthquake foci over the past 40 ? 5 years (figure 28). This result is not dissimilar to that of Dahren and others (2010), but ?pièák and others (2011) concluded that the pattern reflects "depth variations of tectonic stress concentration rather than vertical migration of the source of magma." Similar to Jaxybulatov and others (2011), ?pièák and others (2011) also reported that the deepest events (~ 100 km) constrain the primary magma generation of the system to depths greater than 100 km. They further noted an aseismic gap in the Wadati-Benioff zone of the subducting plate at 100-120 km depth. They inferred that this gap is likely the consequence of partial melting "inhibiting stress concentration necessary to generate stronger earthquakes," while an increased occurrence of earthquakes in the lithospheric wedge above probably reflects magma ascent.

Figure 28. Teleseismic earthquake foci located at Krakatau during 1964-2005. Each panel (depth vs horizontal position) indicates the overall cluster of earthquakes, but emphasizes the earthquakes within the designated time interval (symbols in red). The panels are oriented trending N30W. Open and red symbols represent body-wave magnitudes (mb) as shown in the legend at bottom. From ?pièák and others (2011).

Revised first millenium eruption date.Prior to 2000, a first millennium AD catastrophic and caldera-forming event at Krakatau ("Proto-Krakatau") was hypothesized to have occurred circa 416 AD. From an intensive, multi-disciplinary approach, Wohletz (2000) hypothesized that the catastrophic event actually occurred in the year prior to widespread climate perturbation during 535-536 AD. He noted that the 338th century of the Shaka calendar was probably previously mis-aligned to 416 AD of the western calendar. Since Wohletz (2000), the date of this caldera- forming eruption is widely accepted as 535 AD (see inset of figure 23 for map representation of the caldera's topographic margin).

Our previous report (BGVN 36:08) discussed two eruption episodes: one from 25 October 2010 to March 2011, and another from August 2011 to about 1 October 2011. During the last two weeks of September 2011, the volcano produced persistent volcanic earthquake swarms and thin emissions (BGVN 36:08). This report discusses two visits to the volcano in 2011. Scientists that visited on 8 October 2011 reported degassing and an ongoing seismic swarm then consisting chiefly of M ~1 and smaller earthquakes. During 12-13 November 2011 a photographer noted steady degassing, then observed the start of a 12-hour interval of minor but repeated Stombolian eruptions (see next section).

2011 visits by Øystein Lund Andersen. The photographer and guide Øystein Lund Andersen lives in Jakarta, Indonesia and visits Anak Krakatau often. His website contains photos of the volcano. He shows one photo of a seismograph at CVGHM’s Pasauran Observatory recording part of a prolonged swarm of small earthquakes from 8 October 2011. Youtube features a video he took on the same subject.

His visit to Anak Krakatau during 12-13 November 2011 took place during an interval of gas emissions devoid of ash. He stayed up all night to observe Anak Krakatau emit a steady, white, ash-free plume. At dusk on 12 November he noticed that the crater glowed bright red and after a few hours a series of mild Strombolian eruptions occurred in a sequence that lasted 12 hours (figure 29). The time between the eruptions was from 30 seconds to a few minutes. Some of Andersen’s photos captured glowing pyroclasts arcing tens of meters above the crater rim (figure 29b, c). Anderson saw ash lava bombs in the plume during these eruptions. He noted that the lava bombs ejected over the crater mainly fell back into the crater. During the night the crater remained almost constantly illuminated by the glowing bombs and the fragments they created when they landed. The eruptions were often accompanied by loud sounds from the volcano.

Background. See earlier Bulletin reports for maps of the Krakatau complex and of the post-collapse cone that formed an island and now continues as the active vent (Anak Krakatau, Daughter of Krakatau; for example, figure 23 in BGVN 36:08). Krakatau sits ~130 km W of the Indonesian capital, Jakarta. The complex is famous for the devastating caldera-forming eruption in 1883 (Simkin and Fiske, 1983). That eruption injected millions of tons of fine ash, aerosols, and sulfate particles into the atmosphere. That eruption and associated tsunami claimed over 36,000 lives and awakened the world to caldera collapse (Self and Rampino, 1981).

Lockwood and Hazlett (2010) noted that the 1883 eruption “impressed European observers with remarkable, smog-like sunsets and silvery midday skies. This inspired a number of paintings, possibly including the lurid sky in Edvard Munch’s famous work The Scream, which he painted in 1893.”

According to the Center of Volcanology and Geological Hazard Mitigation (CVGHM), between the emergence of Anak Krakatau from the sea surface on 11 June 1927 up to 2011, the volcano had undergone over 100 eruptions. During that period, the volcano’s non-eruptive periods lasted between 1 and 6 years. During the past few years, Anak Krakatau underwent several eruptive phases, followed by relatively quiet phases (BGVN 34:05, 34:11, and 36:08).

One of our previous reports on Krakatau (BGVN 36:08) discussed two eruption episodes, one spanning from 25 October 2010 to March 2011 and another beginning in August 2011 and continuing through the end of that report, around 1 October 2011. During the last two weeks of September 2011 volcanic earthquake swarms and diffuse emissions persisted. In November 2011, the photographer and guide Øystein Lund Andersen visited Krakatau and observed mild Strombolian explosions (BGVN 37:11).

This report summarizes behavior chiefly during 1 October 2011 through early October 2012. Eruptions around early September 2012 deposited ash on towns in Sumatra, and lava flows extended the shoreline of the island (Anak Krakatau) by ~100 m. The Alert Level was lowered after that and remained low into at least mid-January 2013.

Figure 30 is an index map showing the location of the famous caldera Krakatau, located in the Sunda Strait (E of Sumatra and W of Java). The smaller feature Anak Krakatau grew to form an island well after the 1883 eruption and continues as the active center.

Figure 30. An index map showing the location of Krakatau. Small yellow triangles indicate other Holocene volcanoes (from the current Global Volcanism Program database). Created by GVP staff from ARC software.

October 2011-2012. The Alert Level was raised to 3 (on a scale of 1-4) on 30 September 2011, prohibiting visitors and residents from approaching within 2 km of the active crater. On 8 October 2011, a Jakarta Post article stated that activity at Krakatau was increasing; the number of seismic events on 6 October was 5,204; on 7 October, 5,543; and on 8 October, 5,883.

On 26 January 2012, CVGHM lowered the Alert Level from 3 to 2, noting that the number of tremor events had decreased significantly. The lowered Alert Level excluded visitors and residents from approaching within 1 km of the active crater.

During 1 June-1 September 2012, CVGHM’s visual observations were often prevented by fog cover. When views were clear during June, observers saw occasional diffuse white plumes above the crater. In June, July, and August, the respective seismic events totaled 1,075, 807, and 2,335. Detailed seismicity during 1 June-2 September 2012 is cataloged in table 8.

Table 8. Type and occurrence of earthquakes and tremor at Krakatau during 1 June-2 September 2012; ‘-’ indicates data not reported. Courtesy of CVGHM.

Seismicity increased on 2 September 2012. CVGHM recorded continuous tremor, and a Strombolian eruption ejected lava 200-300 m above the crater. At times, residents heard booming sounds that rattled windows. On 3 September, the Darwin Volcanic Ash Advisory Centre (VAAC) reported that ash plumes rose to altitudes of 2.4-4.3 km and drifted 35-95 km N. The Jakarta Globe (citing the state news agency Antara News) reported that a dense ash plume had drifted N, reaching a number of areas in Lampung (the most southerly province of Sumatra), and blanketing Lampung’s capital, Bandar Lampung (~75 km NNW of Krakatau; figure 31), with a thin layer of ash.

Figure 31. Sketch map showing Krakatau and Sumatra’s Lampung province (Indonesia) including the capital, Bandar Lampung, the scene of September 2012 ashfall from the post-1927 vent, Anak Krakatau (the small island labeled with a triangle). Adapted from a brochure offered for tourists (Indonesia Destination & Travel Information Guide; baliwww.com).

Ashfall on 2 September prompted officials to recommend that residents and tourists wear masks outside and not venture within 3 km of the volcano. The Jakarta Post indicated that government officials planned to distribute 680,000 masks to residents in a number of affected districts in anticipation of further Krakatau explosions.

According to Antara News, less intense tremor continued on 4 September. A satellite image acquired by NASA’s Earth Observatory on 4 September showed fresh lava flows descending Anak Krakatau’s SE flank, extending the shoreline by about 100 m (figure 32).

Figure 32. A natural-color satellite image of Anak Krakatau acquired by the Advanced Land Imager aboard Earth Observatory-1 on the morning of 4 September 2012. The NASA caption noted that fresh lava flows extended the SE shoreline by ~100 m. The ash plume drifted W. Courtesy of NASA Earth Observatory.

September 2012 eruption and visit by Volcano Discovery. A Volcano Discovery group toured Krakatau during the first several days of September 2012 and observed what they noted was the largest explosion there in ten years. They noted that seismic activity (recorded by CVGHM) peaked on 2 September, with a day of continuous explosions and lava descending the volcano’s E and W flanks. Photos showed lava entering the sea (figure 33).

Figure 33. A lava flow from Anak Krakatau reaching the sea on 3 September 2012. Courtesy of Volcano Discovery.

The Volcano Discovery group noted that the eruption had altered part of the S and W crater rim, splitting the rim into two parts. They saw an abundance of lava bombs on the volcano’s flanks in the forest, indicating the force of the explosions. The group indicated that by 5 September, activity had greatly diminished and incandescence from the crater was absent. On 6 September, Krakatau was calm with limited seismic activity reported by the observatory.

On 8 September, the Jakarta Post also reported that seismic activity had greatly diminished during the previous two days. However, this news account noted that residents in Bandar Lampung still reported ashfall. The article also stated that ash from the eruption had damaged volcano-monitoring equipment.

The Alert Level remained at 2 from 26 January 2012 through at least 3 January 2013.

2012 visits by Øystein Lund Andersen. As indicated in our previous report on Krakatau (BGVN 37:11), the photographer and guide Øystein Lund Andersen has visited Krakatau multiple times and his website contains good descriptions and photos of the volcano during these visits. The following describes his observations during 2012.

On 8 January 2012, during a 3-hour visit, Andersen observed no Strombolian activity, in contrast with his observation on 13 November 2011. On his next visit, during 12-14 February, he reported medium to heavy venting from the crater and fumarolic activity that was more intense than the activity during January. He noted continuous steam-and-gas emissions that rose 100-500 m and incandescence at night, but no eruptions.

During a visit on 6-7 April 2012, Andersen noticed that the S part of the crater was illuminated at night. He further reported that on 7 April, Krakatau started to produce small eruptions from the S part of the crater, the same side as the growing lava dome.

During 6-8 May, Andersen noted that activity at Krakatau had decreased somewhat in the previous several weeks. Steam plumes reached a height of 100-200 m and seemed less intense than during his visit in April. He noted incandescence at night, but it was less intense that the previous month. Andersen reported on 7 April: “footage taken by Pierre Fortine showed no sign of any lava dome, but the red glow that is often clearly visible at night from Verlaten, Lang, or Rakata are in fact multiple glowing vents (some of them were gas vents that were burning) and red hot material surrounding them. The lava dome that people have claimed to observe [through] February to April may have been destroyed during the last small eruptions that I reported of in April. Shallow earthquake data recorded by the Krakatau Volcano Observatory in Pasauran (PVMG) shows that the level of activity remains on the relative same level as last month.”

During 2-3 June, he reported that activity had decreased since April and May. He stated that incandescence was almost non-observable, and steam plumes only rose 50-100 m on this visit (compared to 200 m in May), and were at times non-existent.

On 3 September 2012, Andersen wrote that he had heard continuous booming noises half way from Java, and that large booming sounds could be heard in the villages of Carita and Anyar (or Anyer), neighboring villages on the W coast of Java, about 50 km ESE of the volcano. As he approached the volcano, he noted a high plume from the main vent and the ejection of lava bombs to heights of up to 300 m. An area on the SE shore also emitted a large steam plume. According to Andersen: “The local crew/guides who joined our group looked very surprised and worried, as we all noticed these major changes. I first thought this was the result of new geothermal activity, but first realized later that this was in fact a new lava flow.” According to Andersen, the lava flow had extended the seashore on the E side by up to 100 m, and the E and W part of the crater walls had experienced a partial collapse (figure 34).

Figure 34. New lava extending the seashore on the E side of Anak Krakatau on 3 September 2012. Courtesy of Øystein Lund Andersen.

On a visit during 6-7 October 2012, Andersen observed no activity other than a weak and irregular steam plume and, at night, “some small spots of glowing lava, near and on the lava flow on the western flank.” On this visit, he studied the new lava flow. He reported that the lava had flowed down both the W and E sides of the volcano, leaving deep scars on both flanks. According to Andersen, news accounts had reported the lava flow on the E flank, but few had noted the one on the W flank, which was significant, although not as great as the one on the E flank.

Geological Background

The renowned volcano Krakatau (frequently misstated as Krakatoa) lies in the Sunda Strait between Java and Sumatra. Collapse of the ancestral Krakatau edifice, perhaps in 416 CE, formed a 7-km-wide caldera. Remnants of this ancestral volcano are preserved in Verlaten and Lang Islands; subsequently Rakata, Danan and Perbuwatan volcanoes were formed, coalescing to create the pre-1883 Krakatau Island. Caldera collapse during the catastrophic 1883 eruption destroyed Danan and Perbuwatan volcanoes, and left only a remnant of Rakata volcano. This eruption, the 2nd largest in Indonesia during historical time, caused more than 36,000 fatalities, most as a result of devastating tsunamis that swept the adjacent coastlines of Sumatra and Java. Pyroclastic surges traveled 40 km across the Sunda Strait and reached the Sumatra coast. After a quiescence of less than a half century, the post-collapse cone of Anak Krakatau (Child of Krakatau) was constructed within the 1883 caldera at a point between the former cones of Danan and Perbuwatan. Anak Krakatau has been the site of frequent eruptions since 1927.

This compilation of synonyms and subsidiary features may not be comprehensive. Features are organized into four major categories: Cones, Craters, Domes, and Thermal Features. Synonyms of features appear indented below the primary name. In some cases additional feature type, elevation, or location details are provided.

Synonyms

Cracatoa | Krakatao | Krakatoa

Cones

Feature Name

Feature Type

Elevation

Latitude

Longitude

Anak Krakatau

Cone

280 m

Perbuwatan
Perboewatan

Former cone

Rakata

Stratovolcano

813 m

Photo Gallery

An ash-rich eruption column rises above the 1978 crater of Anak Krakatau on September 12, 1979. Explosive activity that began in mid-July 1979 continued until November. A lava flow was emplaced during September 18-21. Verlaten Island, part of the rim of Krakatau caldera, appears in the background to the NW.

The renowned Krakatau volcano lies in the Sunda Strait between Sumatra and Java. The historic eruption of 1883 destroyed much of Krakatau Island, forming a submarine caldera and producing detonations that were heard as far away as Australia. Rakata Island in the background is the truncated rim of the 1883 caldera. Anak Krakatau (Child of Krakatau) in the foreground is a post-caldera cone that first breached the surface of the sea in 1928 and has been in frequent activity since then. The black lava flow at the right side of the photo was erupted in 1975.

Photo courtesy Volcanological Survey of Indonesia, 1979.

The noted Dutch geologist R.D.M. Verbeek conducted a comprehensive study of the 1883 Krakatau eruption. This chromolithograph from his 1885 monograph shows the remnant of Krakatau Island seven weeks after the eruption. The truncated cliff revealing the stratigraphy of Rakata volcano marks the margin of the newly formed Krakatau caldera. Lighter-colored materials capping the outer flanks are pyroclastic-flow deposits from the 1883 eruption.

Chromolithograph published in Verbeek (1885) and Simkin and Fiske, 1993.

One week after the onset of an eruption of Krakatau volcano in May 1883, an eruption column rises above Perboewatan vent on Krakatau Island. Initially three vents on Krakatau Island were active. Three months later paroxysmal eruptions destroyed much of the island, creating a large caldera. Pyroclastic flows swept across the sea to Sumatra, and tsunamis swept the coastlines of Sumatra and Java.

Photo courtesy of the family of R. Breon, published in Simkin and Fiske (1983).

This large block of coral was torn from offshore reefs and thrown inland near Anjer, NW Java (about 55 km from Krakatau), by the powerful tsunami produced during the 1883 eruption. The 300 cu m coral block (note person at right for scale) weighed approximately 600 tons.

Photo from Royal Institute for the Tropics, Amsterdam, 1886 (published in Simkin and Fiske, 1983).

The steamship Berouw, torn from its mooring in Telok Betok harbor in SE Sumatra, was swept inland by the tsunami from the 1883 eruption and marooned almost intact in the valley of the Koeripan River, 3.3 km from its anchorage. The ship was never refloated. The boiler remained visible for almost a century, but after a heavy flood in 1979 carried it 1 km downstream, it was cut up for scrap metal.

Etching by T. Weber (published in Simkin and Fiske, 1983).

Optical effects of the 1883 eruption were noted around the world. A series of pastels by William Ascroft documented atmospheric effects such as this lurid November 16, 1883 sunset at Chelsea, London. In addition to extraordinarily long and vivid sunsets, skies became progessively more brilliant after the cessation of normal twilight.

Pastel by William Ascroft, 1883 (from the Science Museum, London, published in Simkin and Fiske, 1993).

A boatload of visitors prepares to sail to Anak Krakatau Island, seen behind the mast of the boat. Anak Krakatau was constructed within the 1883 caldera, whose southern rim forms the conical Rakata peak in the right background.

Photo by Dick Fiske, 1983 (Smithsonian Institution).

An eruption column rises above Peraboewatan crater on Krakatau Island on May 27, 1883. Three months later one of history's most noted eruptions destroyed much of the island, forming a submarine caldera. Detonations were heard as far away as Australia, pyroclastic flows swept across the sea to the coast of Sumatra, and powerful tsunamis devastated the shores of Sumatra and Java.

Photo courtesy Volcanological Survey of Indonesia, 1883.

Submarine eruptions within Krakatau caldera were first observed in December, 1927, and an ephemeral island appeared the following month. This 1929 view shows a cockscomb-like ash-rich eruption column typical of shallow submarine explosions. Material ejected by earlier eruptions forms an island visible to the left; by August 1930 Anak Krakatau (Child of Krakatau) became a permanent island.

A dark, ash-laden cloud is ejected from a submarine vent at Anak Krakatau (Child of Krakatau) on June 12, 1930. A white steam column rises above a pyroclastic-surge that travels horizontally along the sea surface in a radial direction from the vent. "Base surges" such as these are a common phenomenon of submarine eruptions. The first eruptions of Anak Krakatau to breach the surface were seen in December 1927.

A submarine eruption from Anak Krakatau on June 13, 1930, produces both a vertical eruption column and a horizontally traveling "base surge" extending radially from the vent along the surface of the sea. Submarine eruptions were first observed in December 1927, forming several ephemeral islands. By August 12, 1930, Anak Krakatau (Child of Krakatau) became a permanent island.

An eruption column rises above Anak Krakatau on February 13, 1960. Explosive activity at Anak Krakatau was almost continuous from December 1959 until 1963. Construction of a new cinder cone displaced the crater lake and produced a subaerial vent. A lava flow that reached the sea was emplaced sometime between 1960 and 1963.

Photo courtesy Volcanological Survey of Indonesia, 1960.

An eruption column rises above a cone in the summit crater of Anak Krakatau on May 2, 1961. Anak Krakatau was in almost continuous activity from December 1959 until 1963. The newly formed cinder cone displaced a lake that had filled the crater prior to the start of the eruption.

Photo by D. Hadikusumo, 1961 (Volcanological Survey of Indonesia).

An eruption plume rises above Anak Krakatau on January 4, 1973. An eruption that began in June 1972 lasted until mid-1973. Frequent ash eruptions from June to September 1972 were followed in December by renewed explosive activity and the effusion of a lava flow, which ceased flowing in January 1973.

Photo by S. Wikartadipura, 1973 (Volcanological Survey of Indonesia).

Explosive eruptions, such as this one in October 1978, occurred at Anak Krakatau from July to November, 1978. Initially, explosions occurred at intervals of 15-30 minutes, decreasing to intervals of 30-60 minutes in October before the eruption ended in November.

Photo by J. Matahelumual, 1978 (Volcanological Survey of Indonesia).

A small dark eruption plume begins rising from the crater of Anak Krakatau in October 1978. Explosive activity began in July and lasted until November. A blocky lava flow from a previous eruption forms the coast in the foreground.

Photo by J. Matahelumual, 1978 (Volcanological Survey of Indonesia).

An explosive eruption in 1979 from Anak Krakatau is seen in an aerial view from the north. The island of Anak Krakatau (Child of Krakatau), a post-caldera cone that has grown within the submarine caldera of 1883, first breached the surface in 1928.

Photo by Adjat Sudradjat, 1979 (Volcanological Survey of Indonesia).

Anak Krakatau is seen here in 1993 from the east, with the 1993 lava flow reaching the sea in the foreground. This eruption began with explosions and lava emission on November 7, 1992. Lava flowed to the SE and NE, eventually reaching the NW coast. Another lava flow traveled to the SSE beginning in February 1993 and reached the southern coast. A third flow descended to the north in April and May. Explosive activity was continuing in June, when a tourist was killed and five others injured. Eruptions continued until October 1993.

Photo by Ruska Hadian, 1993 (Volcanological Survey of Indonesia).

An eruption plume rising above Anak Krakatau on March 2, 1988, was part of an explosive and effusive eruption from mid-February until April. The eruption took place from a fissure on the SSE side of the 1960-61 cinder cone.

Photo by Klaus Mehl, 1988 (Ruhr University, Germany).

Intermittent explosions, such as this one on March 2, 1988, took place from a fissure on the SSE side of the 1960-1981 cone of Anak Krakatau from mid-February to April 1988. Two small lava flows were also emitted during late February and March, accompanied by small explosions.

Photo by Klaus Mehl, 1988 (Ruhr University, Germany).

An ash plume from Anak Krakatau, 1 November 1, 2007 is seen from the NW from a monitoring station on Sertung Island. A detached ash plume from an earlier explosion drifts to the left as a new plume rises above the vent. Rakata Island is in the background. Explosive activity had resumed at Krakatau on October 23. Ash eruptions and ejection of incandescent material were accompanied by the effusion of lava flows.

References

The following references have all been used during the compilation of data for this volcano, it is not a comprehensive bibliography. Discussion of another volcano or eruption (sometimes far from the one that is the subject of the manuscript) may produce a citation that is not at all apparent from the title.

WOVOdat is a database of volcanic unrest; instrumentally and visually recorded changes in seismicity, ground deformation, gas emission, and other parameters from their normal baselines. It is sponsored by the World Organization of Volcano Observatories (WOVO) and presently hosted at the Earth Observatory of Singapore.

EarthChem develops and maintains databases, software, and services that support the preservation, discovery, access and analysis of geochemical data, and facilitate their integration with the broad array of other available earth science parameters. EarthChem is operated by a joint team of disciplinary scientists, data scientists, data managers and information technology developers who are part of the NSF-funded data facility Integrated Earth Data Applications (IEDA). IEDA is a collaborative effort of EarthChem and the Marine Geoscience Data System (MGDS).